算法链表链表相关问题总结
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剑指offer
6. 从尾到头打印链表
题目链接：https://leetcode-cn.com/problems/cong-wei-dao-tou-da-yin-lian-biao-lcof/
递归
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
vector<int> reversePrint(ListNode* head) {
if(!head)
return {};
vector<int> v = reversePrint(head->next);
v.push_back(head->val);
return v;
}
};
栈
class Solution {
public:
vector<int> reversePrint(ListNode* head) {
vector<int> v;
ListNode* cur = head;
while (cur) {
v.push_back(cur->val);
cur = cur->next;
}
reverse(v.begin(), v.end());
return v;
}
};
reverse数组
class Solution {
public:
vector<int> reversePrint(ListNode* head) {
vector<int> v;
ListNode* cur = head;
while (cur) {
v.push_back(cur->val);
cur = cur->next;
}
reverse(v.begin(), v.end());
return v;
}
};
18. 删除链表的节点
题目链接：https://leetcode-cn.com/problems/shan-chu-lian-biao-de-jie-dian-lcof/
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* deleteNode(ListNode* head, int val) {
auto dummy = new ListNode(-1);
dummy -> next = head;
auto tmp = dummy;
while(head && head -> val != val)
{
tmp = tmp -> next;
head = head -> next;
}
// cout << tmp -> val <<endl;
// cout << head -> val <<endl;
tmp -> next = head -> next;
return dummy -> next;
}
};
22. 链表中倒数第k个节点
题目链接：https://leetcode-cn.com/problems/lian-biao-zhong-dao-shu-di-kge-jie-dian-lcof/
快慢指针
快指针比慢指针快k个节点，当快指针指向尾结点的下一个节点的时候，慢指针指向倒数第k个节点
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* getKthFromEnd(ListNode* head, int k) {
ListNode* fast = head;
ListNode* low = head;
while(fast){
fast = fast -> next;
if(k > 0) k--;
else low = low -> next;
}
return low;
}
};
24. 反转链表
题目链接：https://leetcode-cn.com/problems/fan-zhuan-lian-biao-lcof/
思路
依次遍历链表，遍历一个逆置一个。
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* reverseList(ListNode* head) {
ListNode *new_head = nullptr;
while(head)
{
ListNode *next = head->next;
head->next = new_head;
new_head = head;
head = next;
}
return new_head;
}
};
25. 合并两个排序的链表
题目链接：https://leetcode-cn.com/problems/he-bing-liang-ge-pai-xu-de-lian-biao-lcof/
使用一个
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
ListNode* new_head = new ListNode(-1); //保存合并后的头结点
ListNode* tmp = new_head; //遍历链表
while(l1 && l2)
{
//小的追加到tmp后面
if(l1 -> val <= l2 -> val)
{
tmp -> next = l1;
l1 = l1 -> next;
}
else
{
tmp -> next = l2;
l2 = l2 -> next;
}
//更新tmp
tmp = tmp -> next;
}
//没处理完的直接追加到后面
tmp -> next = l1 ? l1 : l2;
return new_head -> next;
}
};
35. 复杂链表的复制
题目链接：https://leetcode-cn.com/problems/fu-za-lian-biao-de-fu-zhi-lcof/
哈希
用哈希表做一个映射，hashmap的key是原先的节点，val是复制的节点。先初始化hashmap，然后分别next指针和random指针。
哈希表的作用：找到每个点的对应点（复制点）
时间复杂度O(n) 空间复杂度O(n)
/*
// Definition for a Node.
class Node {
public:
int val;
Node* next;
Node* random;
Node(int _val) {
val = _val;
next = NULL;
random = NULL;
}
};
*/
class Solution {
public:
Node* copyRandomList(Node* head) {
if(!head) return head;
unordered_map<Node*, Node*> hashmap;
hashmap[NULL] = NULL;
//初始化hashmap
//hashmap key是原先的节点p val是新节点
for(auto p = head; p; p = p -> next)
hashmap[p] = new Node(p -> val);
for(auto p = head; p; p = p -> next)
{
//复制next指针
hashmap[p] -> next = hashmap[p -> next];
//复制random指针
hashmap[p] -> random = hashmap[p -> random];
}
return hashmap[head];
}
};
迭代
先复制正常的next节点，不复制random节点。之后复制指针的指向
时间复杂度O(n) 空间复杂度O(1)
*
// Definition for a Node.
class Node {
public:
int val;
Node* next;
Node* random;
Node(int _val) {
val = _val;
next = NULL;
random = NULL;
}
};
*/
class Solution {
public:
Node* copyRandomList(Node* head) {
//在每个节点后面添加它的复制
//复制next节点 复制的节点保存在原先节点的next指针处
//新复制的节点的next指针的原先节点的next指针
//原先： 1->2->3->4->5->null
//now： 1->1->2->2->3->3->4->4->5->5->null
for(auto p = head; p;)
{
auto np = new Node(p -> val);
auto next = p -> next;
p -> next = np;
np -> next = next;
p = next;
}
//复制random指针
for(auto p = head; p; p = p -> next -> next)
if(p -> random)
p -> next -> random = p -> random -> next;
//原链表不能修改 还原
//遍历链表 分开两个链表
auto dummy = new Node(-1);
auto cur = dummy;
for(auto p = head; p; p = p -> next)
{
cur -> next = p -> next;
cur = cur -> next;
p -> next = p -> next -> next;
}
return dummy -> next;
}
};
36. 二叉搜索树与双向链表
题目链接：https://leetcode-cn.com/problems/er-cha-sou-suo-shu-yu-shuang-xiang-lian-biao-lcof/
递归
Leetcode要求改成双向循环链表
剑指offer是改成双向链表
题目链接：https://www.acwing.com/problem/content/87/
left 改成 前驱
right 改成后继
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
TreeNode* convert(TreeNode* root) {
if (!root) return NULL;
auto sides = dfs(root);
return sides.first;
}
pair<TreeNode*, TreeNode*> dfs(TreeNode* root)
{
//叶子节点
if (!root->left && !root->right) return {root, root};
//左右子树都有
if (root->left && root->right)
{
//递归处理左右子树
auto lside = dfs(root->left), rside = dfs(root->right);
//拼接左右子树
lside.second->right = root, root->left = lside.second;
root->right = rside.first, rside.first->left = root;
return {lside.first, rside.second};
}
//只有左子树
if (root->left)
{
auto lside = dfs(root->left);
lside.second->right = root, root->left = lside.second;
return {lside.first, root};
}
//只有右子树
if (root->right)
{
auto rside = dfs(root->right);
rside.first->left = root, root->right = rside.first;
return {root, rside.second};
}
}
};
Leetcode
92. 反转链表II
题目链接：https://leetcode-cn.com/problems/reverse-linked-list-ii/
几个关键节点：
1. 逆置前头节点->逆置后尾节点
2. 逆置前尾节点->逆置后头节点
3. 逆置前头节点的前驱
4. 逆置前尾节点的后继
思路：
- 找到开始逆置的节点，记录该节点的前驱和该节点；
- 从head开始，逆置n-m+1个节点；
- 将pre_head与new_head连接，aft_tail与head连接。
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* reverseBetween(ListNode* head, int m, int n) {
int len = n - m + 1; // 需要写在改变m之前
ListNode* res = head; //最终的头节点
ListNode *pre_head = NULL;//开始逆置的节点的前驱
while(head && --m)
{
pre_head = head; //head的前驱
head = head -> next; //head向后移动m-1个位置
}
ListNode* aft_tail = head; //逆置后的尾节点
ListNode* new_head = NULL; //逆置后的头节点
while(head && len) //逆置m到n
{
ListNode *next = head -> next;
head -> next = new_head;
new_head = head;
head = next;
len--;
}
aft_tail -> next = head; //逆置后尾节点的下一个节点是逆置前尾节点的后继
if(pre_head) pre_head -> next = new_head; //逆置节点的前驱部位空，第一个逆置的不是头节点
else res = new_head;//从头节点开始逆置的
return res;
}
};
146. LRU缓存
题目链接：https://leetcode-cn.com/problems/lru-cache/solution/lru-ce-lue-xiang-jie-he-shi-xian-by-labuladong/
思路：
使用两个双链表和一个哈希表：第一个双链表存储未被使用的位置；第二个双链表存储已被使用的位置，且按最近使用时间从左到右排好序；哈希表存储key对应的链表中的节点地址；
class LRUCache {
public:
struct Node
{
int val, key;
Node *left, *right;
Node() : key(0), val(0), left(NULL), right(NULL) {}
};
Node *hu, *tu; // hu: head_used, tu: tail_used; head在左侧，tail在右侧
Node *hr, *tr; // hr: head_remains, tr: tail_remains; head在左侧，tail在右侧
int n;
unordered_map<int, Node*> hash;
void delete_node(Node *p)
{
p->left->right = p->right, p->right->left = p->left;
}
void insert_node(Node *h, Node *p)
{
p->right = h->right, h->right = p;
p->left = h, p->right->left = p;
}
/*初始化*/
LRUCache(int capacity) {
n = capacity;
hu = new Node(), tu = new Node();
hr = new Node(), tr = new Node();
hu->right = tu, tu->left = hu;
hr->right = tr, tr->left = hr;
//第一个双链表插入 n 个节点，n 是缓存大小；
//第二个双链表和哈希表都为空；
for (int i = 0; i < n; i ++ )
{
Node *p = new Node();
insert_node(hr, p);
}
}
int get(int key) {
if (hash[key]) //用哈希表判断key是否存在
{
Node *p = hash[key];
delete_node(p);
insert_node(hu, p);
return p->val; //如果key存在，则返回对应的value，同时将key对应的节点放到第二个双链表的最左侧；
}
return -1; //如果key不存在，则返回-1
}
void put(int key, int value) {
if (hash[key]) //用哈希表判断key是否存在
{
//key存在，则修改对应的value，同时将key对应的节点放到第二个双链表的最左侧；
Node *p = hash[key];
delete_node(p);
insert_node(hu, p);
p->val = value;
return;
}
//key不存在：
//如果缓存已满，则删除第二个双链表最右侧的节点（上次使用时间最老的节点），同时更新三个数据结构；
if (!n)
{
n ++ ;
Node *p = tu->left;
hash[p->key] = 0;
delete_node(p);
insert_node(hr, p);
}
//否则，插入(key, value)：从第一个双链表中随便找一个节点，修改节点权值，然后将节点从第一个双链表删除，插入第二个双链表最左侧，同时更新哈希表；
n -- ;
Node *p = hr->right;
p->key = key, p->val = value, hash[key] = p;
delete_node(p);
insert_node(hu, p);
}
};
/**
* Your LRUCache object will be instantiated and called as such:
* LRUCache obj = new LRUCache(capacity);
* int param_1 = obj.get(key);
* obj.put(key,value);
*/
使用一个双链表和一个哈希表
class LRUCache {
public:
LRUCache(int capacity) {
cap = capacity;
used = 0;
head = new Node(0, 0);
tail = new Node(0, 0);
head->next = tail;
tail->pre = head;
}
int get(int key) {
if (m.count(key)) {
Node *p = m[key];
int val = p->val;
delete_node(p);
m[key] = insert_node(key, val);
return val;
}
return -1;
}
void put(int key, int value) {
if (m.count(key)) {
delete_node(m[key]);
used--;
}
m[key] = insert_node(key, value);
used++;
if (used > cap) {
m.erase(tail->pre->key);
delete_node(tail->pre);
used--;
}
}
private:
int cap, used;
struct Node{
int val, key;
Node *pre, *next;
Node(int key, int val): key(key), val(val), pre(NULL), next(NULL){};
};
unordered_map<int, Node*> m;
Node *head, *tail;
void delete_node(Node *p) {
p->pre->next = p->next;
p->next->pre = p->pre;
delete p;
}
Node* insert_node(int key, int val) {
Node *node = new Node(key, val);
head->next->pre = node;
node->next = head->next;
head->next = node;
node->pre = head;
return node;
}
};
/**
* Your LRUCache object will be instantiated and called as such:
* LRUCache obj = new LRUCache(capacity);
* int param_1 = obj.get(key);
* obj.put(key,value);
*/
147. 链表插入排序
题目链接：https://leetcode-cn.com/problems/insertion-sort-list/
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* insertionSortList(ListNode* head) {
//借助dummy指针
ListNode* dummy = new ListNode(-1);
while(head)
{
ListNode* next = head -> next;
ListNode* p = dummy;
//找到待插入元素的合适位置
while(p -> next && p -> next -> val <= head -> val) p = p -> next;
//将待插入元素出入到链表中
head -> next = p -> next;
p -> next = head;
head = next;
}
return dummy -> next;
}
};
148. 排序链表
题目链接：https://leetcode-cn.com/problems/sort-list/
快速排序
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
//得到尾节点
ListNode* get_tail(ListNode* head){
while(head -> next) head = head -> next;
return head;
}
ListNode* sortList(ListNode* head) {
if(!head || !head->next) return head;
auto left = new ListNode(-1), mid = new ListNode(-1), right = new ListNode(-1);
auto ltail = left, mtail = mid, rtail = right;
int val = head -> val;
for(auto p = head; p; p = p -> next)
{
if(p -> val < val) ltail = ltail -> next = p;
else if(p -> val == val) mtail = mtail -> next = p;
else rtail = rtail -> next = p;
}
ltail -> next = mtail -> next = rtail -> next = NULL;
left -> next = sortList(left -> next);
right -> next = sortList(right -> next);
//拼接三个链表
get_tail(left) -> next = mid -> next;
get_tail(left) -> next = right -> next;
auto p = left -> next;
// delete left;
// delete mid;
// delete right;
return p;
}
};
归并排序
class Solution {
public:
ListNode* sortList(ListNode* head) {
if(!head || !head->next) return head;
ListNode* pre = head, *slow = head, *fast = head;
while(fast && fast->next) {
pre = slow;
slow = slow->next;
fast = fast->next->next;
}
pre->next = nullptr;
return mergeTwoList(sortList(head), sortList(slow));
}
ListNode* mergeTwoList(ListNode* h1, ListNode* h2) {
if(!h1) return h2;
if(!h2) return h1;
if(h1->val < h2->val) {
h1->next = mergeTwoList(h1->next, h2);
return h1;
}
else {
h2->next = mergeTwoList(h1, h2->next);
return h2;
}
}
};
143. 重排链表
题目链接：https://leetcode-cn.com/problems/reorder-list/
思路：
- 将后半段的指针都反向；
- 用两个指针分别从1和n开始往中间扫描，将后半段交替插入到前半段。
时间复杂度分析：整个链表总共扫描三次，第一次求总长度，第二次将后半段反向，第三次将后半段交替插入前半段，所以总时间复杂度是 O(n)。
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
void reorderList(ListNode* head) {
//获取链表长度
int n = 0;
for (ListNode *p = head; p; p = p->next) n ++ ;
//长度小于2，不用重排
if (n <= 2) return;
//获取节点
ListNode *later = head;
for (int i = 0; i + 1 < (n + 1) / 2; i ++ )
later = later->next;
//逆置后半串节点
ListNode *a = later, *b = later->next;
while (b)
{
ListNode *c = b->next;
b->next = a;
a = b;
b = c;
}
//依次将后部分的节点插入到前面正确的位置
later->next = 0;
while (head && head != a)
{
b = a->next;
a->next = head->next;
head->next = a;
head = head->next->next;
a = b;
}
}
};
141. 环形链表
题目链接：https://leetcode-cn.com/problems/linked-list-cycle/
快慢指针
快指针一次走两步，慢指针一次走一步，当两个指针第一次相遇的时候，快和慢指针一次都走一步，第二次相遇的点就是环的入口。可以证明
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
bool hasCycle(ListNode *head) {
if(!head || !head -> next) return nullptr;
ListNode* fast = head;
ListNode* low = head;
while(fast && low)
{
fast = fast -> next;
low = low -> next;
if(fast) fast = fast -> next;
//如果快指针走到nullptr，说明链表没有环
else return false;
//第一次相遇后，令fast = head 两个节点都走一步
if(fast == low)
{
fast = head;
while(fast != low)
{
fast = fast -> next;
low = low -> next;
}
return true;
}
}
return false;
}
};
142 环形链表II
题目链接：
160. 相交链表
题目链接：https://leetcode-cn.com/problems/intersection-of-two-linked-lists/
双指针
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
if(!headA || !headB) return nullptr;
auto A = headA, B = headB;
while(A != B)
{
A = A -> next;
B = B -> next;
if(!A && !B) return nullptr;
if(!A) A = headB;
if(!B) B = headA;
}
return A;
}
};
86. 分隔链表
题目链接：https://leetcode-cn.com/problems/partition-list/
快速排序中的一步，将元素放到合适的位置，左边比他小，右边比他大或者相等。
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
//得到left的尾结点
ListNode* get_tail(ListNode* head){
while(head -> next) head = head -> next;
return head;
}
ListNode* partition(ListNode* head, int x) {
if(!head) return head;
auto left = new ListNode(-1), right = new ListNode(-1);
auto ltail = left, rtail = right;
for(auto p = head; p; p = p -> next)
{
if(p -> val < x) ltail = ltail -> next = p;
else rtail = rtail -> next = p;
}
ltail -> next = rtail -> next = nullptr;
get_tail(left) -> next = right -> next;
return left -> next;
}
};
23. 合并K个有序的链表
题目链接：https://leetcode-cn.com/problems/merge-k-sorted-lists/
顺序合并
class Solution {
public:
ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
ListNode* new_head = new ListNode(-1); //保存合并后的头结点
ListNode* tmp = new_head; //遍历链表
while(l1 && l2)
{
//小的追加到tmp后面
if(l1 -> val <= l2 -> val)
{
tmp -> next = l1;
l1 = l1 -> next;
}
else
{
tmp -> next = l2;
l2 = l2 -> next;
}
//更新tmp
tmp = tmp -> next;
}
//没处理完的直接追加到后面
tmp -> next = l1 ? l1 : l2;
return new_head -> next;
}
ListNode* mergeKLists(vector<ListNode*>& lists) {
ListNode *ans = nullptr;
for (size_t i = 0; i < lists.size(); ++i) {
ans = mergeTwoLists(ans, lists[i]);
}
return ans;
}
};
分治合并
class Solution {
public:
ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
ListNode* new_head = new ListNode(-1); //保存合并后的头结点
ListNode* tmp = new_head; //遍历链表
while(l1 && l2)
{
//小的追加到tmp后面
if(l1 -> val <= l2 -> val)
{
tmp -> next = l1;
l1 = l1 -> next;
}
else
{
tmp -> next = l2;
l2 = l2 -> next;
}
//更新tmp
tmp = tmp -> next;
}
//没处理完的直接追加到后面
tmp -> next = l1 ? l1 : l2;
return new_head -> next;
}
ListNode* merge(vector<ListNode*>& lists, int l, int r)
{
if(l == r) return lists[l];
if(l > r) return nullptr;
int mid = (l + r) >> 1;
return mergeTwoLists(merge(lists, l, mid), merge(lists, mid + 1, r));
}
ListNode* mergeKLists(vector<ListNode*>& lists) {
return merge(lists, 0, lists.size() - 1);
}
};
234. 回文链表
题目链接：https://leetcode-cn.com/problems/palindrome-linked-list/
逆置链表
逆置链表的后半部分然后比较
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
bool isPalindrome(ListNode* head) {
if(!head || !head -> next) return true;
auto slow = head, fast = head;
while(fast && fast -> next)
{
slow = slow -> next;
fast = fast -> next -> next;
}
fast = nullptr; //逆置后的头节点 逆置前的尾节点
while(slow)
{
auto next = slow -> next;
slow -> next = fast;
fast = slow;
slow = next;
}
while(head && fast)
{
if(head -> val != fast -> val) return false;
head = head -> next;
fast = fast -> next;
}
delete slow;
delete fast;
return true;
}
};
小技巧
fast一次走2步，slow一次走1步，链表全长为len，len为偶数时，slow到达前半段的最后一个节点，len为奇数时，slow到达正中间的节点，两种情况中，slow->next均为后半段的起始节点。
ListNode* getMid(ListNode* head){
ListNode* fast = head, * slow = head;
while(fast && fast -> next){
slow = slow->next;
fast = fast->next->next;
}
delete fast;
return slow;
}
2. 两数相加
题目链接：
19. 删除链表的倒数第N个节点
题目链接：https://leetcode-cn.com/problems/remove-nth-node-from-end-of-list/submissions/
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* removeNthFromEnd(ListNode* head, int n) {
ListNode* dummy = new ListNode(-1);
dummy -> next = head;
auto fast = dummy, low = dummy;
while(fast)
{
fast = fast -> next;
if(n >= 0) n--;
else low = low -> next;
}
return dummy -> next;
}
};
21. 合并两个有序链表
题目链接：https://leetcode-cn.com/problems/merge-two-sorted-lists/submissions/
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
ListNode* dummy = new ListNode(-1);
ListNode* tmp = dummy;
while(l1 && l2)
{
if(l1 -> val < l2 -> val)
{
tmp -> next = l1;
l1 = l1 -> next;
}
else
{
tmp -> next = l2;
l2 = l2 -> next;
}
tmp = tmp -> next;
}
tmp -> next = l1 ? l1 : l2;
return dummy -> next;
}
};
876. 链表的中间节点
题目链接：https://leetcode-cn.com/problems/middle-of-the-linked-list/
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* middleNode(ListNode* head) {
auto fast = head, low = head;
while(fast && fast -> next)
{
low = low -> next;
fast = fast -> next -> next;
}
return low;
}
};
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* middleNode(ListNode* head) {
ListNode* dummy = new ListNode(-1);
dummy -> next = head;
auto fast = dummy, low = dummy;
while(fast -> next && fast -> next -> next)
{
low = low -> next;
fast = fast -> next -> next;
}
return low -> next;
}
};
114. 二叉树展开为链表
题目链接：https://leetcode-cn.com/problems/flatten-binary-tree-to-linked-list/
递归
在还没操作节点右子树前，不能破坏该节点的右子树指向。所以采用后序遍历。
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
TreeNode* last = nullptr;
void flatten(TreeNode* root) {
if (root == nullptr) return;
flatten(root->right);
flatten(root->left);
root->right = last;
root->left = nullptr;
last = root;
}
};
迭代(先序遍历)
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
void flatten(TreeNode* root) {
//先序遍历
stack<TreeNode*> node_stack;
if(nullptr != root){
node_stack.push(root);
}
TreeNode* current_node = root;
while(!node_stack.empty()){
TreeNode *node = node_stack.top();
node_stack.pop();
if(nullptr != node->right){
node_stack.push(node->right);
}
if(nullptr != node->left){
node_stack.push(node->left);
}
if(node != root){
current_node->left = nullptr;
current_node->right = node;
}
current_node = node;
}
}
};
总结
链表的问题比较考察逻辑思维，可以通过画图来理清思路。
常用操作
获取链表中间位置的元素
ListNode* getMid(ListNode* head){
ListNode* fast = head, * slow = head;
while(fast && fast -> next){
slow = slow->next;
fast = fast->next->next;
}
delete fast;
return slow;
}
删除倒数第K个节点
ListNode* removeNthFromEnd(ListNode* head, int n) {
ListNode* dummy = new ListNode(-1);
dummy -> next = head;
auto fast = dummy, low = dummy;
while(fast)
{
fast = fast -> next;
if(n >= 0) n--;
else low = low -> next;
}
return dummy -> next;
}
逆置链表
ListNode* reverseList(ListNode* head) {
ListNode *new_head = nullptr;
while(head)
{
ListNode *next = head->next;
head->next = new_head;
new_head = head;
head = next;
}
return new_head;
}
技巧和注意事项
指针丢失和内存泄漏
插入节点
插入之前，应该先把要插入位置之前的元素c的下一个节点保存下来
/*
a -> b -> c -> d -> e
c后面插入f
*/
//写法1
cur = c -> next;
c -> next = f;
f -> next = cur;
//写法2
f -> next = c -> next;
c -> next = f;
删除节点
p -> next = p -> next -> next;
（哨兵）dummy节点
用来解决边界问题，不参与业务逻辑，引入链表以后，不管链表是否为空，dummy->next为头结点；
边界问题
1. 链表为空
2. 链表只包含一个节点
3.链表只包含两个节点
4. 链表对头尾节点的处理
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