LeetCode101.Symmetric Tree

Posted 2020-10-03 高阳阳与朱彦沛的小窝

题目

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

    1
   /   2   2
 / \ / 3  4 4  3

But the following [1,2,2,null,3,null,3] is not:

    1
   /   2   2
   \      3    3

Note:
Bonus points if you could solve it both recursively and iteratively.

题解

题目的意思是，判断一棵二叉树是不是自我镜像的，也就是以中心为轴镜面对称。解题关键是确定镜面节点对，然后判断该对节点值是否一样即可。通过观察可以发现，只要左子树与右子树镜面对称即可，左子树的左孩子与右子树的右孩子配对，左子树的右孩子与右子树的左孩子配对，因而可以递归求解 也可以迭代求解。

（1）递归函数的输入是配对节点，输出是这两个点的值是否相同。如果不相同则返回False，否则需判断这两个节点的左右孩子是否相同。

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     bool isSymmetric(TreeNode* root) {
13         if(root == NULL)
14             return true;
15         return isSymmetric(root->left,root->right);
16     }
17     bool isSymmetric(TreeNode* left,TreeNode* right)
18     {
19         if(left == NULL && right == NULL)  return true;
20         if(left == NULL || right == NULL)    return false;
21         if(left->val != right->val)
22             return false;
23         return isSymmetric(left->left,right->right) && isSymmetric(left->right,right->left);
24     }
25 };

LeetCode101_Recurively

 

（2）迭代解法：借助两个队列，一个代表左子树，一个代表右子树，队列中节点以此对应。

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     bool isSymmetric(TreeNode* root) {
13         if(root == NULL)
14             return true;
15         queue<TreeNode*> lq,rq;
16         lq.push(root->left);
17         rq.push(root->right);
18         while(!lq.empty() && !rq.empty()){
19             TreeNode *tempLeft = lq.front(), * tempRight = rq.front();
20             lq.pop();rq.pop();
21             if(tempLeft == NULL){
22                 if(tempRight == NULL)
23                     continue;
24                 else
25                     return false;
26             }
27             if(tempRight == NULL)
28                 return false;
29             if(tempLeft -> val != tempRight -> val)
30                 return false;
31             lq.push(tempLeft->left);rq.push(tempRight->right);
32             lq.push(tempLeft->right);rq.push(tempRight->left);
33         }
34         if(!lq.empty())
35             return false;
36         if(!rq.empty())
37             return false;
38         return true;
39     }
40     
41 };

Leetcode101_iteratively_1

迭代的时间居然比递归的时间慢了一倍。。用一个队列也是可以模拟的，每次从队首取出两个相互配对的节点即可。

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     bool isSymmetric(TreeNode* root) {
13         if(root == NULL)
14             return true;
15         queue<TreeNode*> q;
16         q.push(root->left);
17         q.push(root->right);
18         while(!q.empty()){
19             TreeNode *tempLeft = q.front(); q.pop();
20             TreeNode *tempRight = q.front(); q.pop();
21             if(tempLeft == NULL && tempRight == NULL) continue;
22             if(tempLeft == NULL || tempRight == NULL) return false;
23             if(tempLeft -> val != tempRight -> val) return false;
24             q.push(tempLeft->left);
25             q.push(tempRight->right);
26             q.push(tempLeft->right);
27             q.push(tempRight->left);
28         }
29         return true;
30     }
31     
32 };

LeetCode101_iteratively_2

至于时间上变慢的原因，目前不知。。