[leetcode]117. Populating Next Right Pointers in Each NodeII用next填充同层相邻节点
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Given a binary tree
struct TreeLinkNode { TreeLinkNode *left; TreeLinkNode *right; TreeLinkNode *next; }
Populate each next pointer to point to its next right node. If there is no next right node, the next pointer should be set to NULL
.
Initially, all next pointers are set to NULL
.
Note:
- You may only use constant extra space.
- Recursive approach is fine, implicit stack space does not count as extra space for this problem.
Example:
Given the following binary tree,
1 / 2 3 / \ 4 5 7
After calling your function, the tree should look like:
1 -> NULL / 2 -> 3 -> NULL / \ 4-> 5 -> 7 -> NULL
题意:
给定一个任意二叉树,其中每个节点都有next指针。
设法将next指针指向同一层右侧相邻节点。
思路:
递归,解法很巧妙。
以 1 为curr,用pre来连接curr.left:2 和 curr.right:3。因为此时根节点 1的next指向null, 退出for循环
1 ( curr )--->null / dummy(-1):pre--> 2---> 3 / \ 4 5 7
递归调用connect(dummy.next),因为dummy.next指向2,此时 2 为curr。用pre来连接curr.left:4 和 curr.right:5。
1 / 2(curr)---> 3 / \ \
dummy(-1):pre--> 4--->5 7
继续走for循环, curr = curr.next, 此时curr为3。 继续用pre连接 curr.left(3的左子树为Null) 和 curr.right:7。
1
/ 2 ---> 3(curr)
/ \ \
dummy(-1):pre--> 4--->5 --->7
因为此时curr的next指向null, 退出for循环。
继续递归调用connect(dummy.next)... 直至 root == null, return
代码:
1 public class Solution { 2 public void connect(TreeLinkNode root) { 3 if (root == null) return; 4 5 TreeLinkNode dummy = new TreeLinkNode(-1); 6 for (TreeLinkNode curr = root, prev = dummy; 7 curr != null; curr = curr.next) { 8 if (curr.left != null){ 9 prev.next = curr.left; 10 prev = prev.next; 11 } 12 if (curr.right != null){ 13 prev.next = curr.right; 14 prev = prev.next; 15 } 16 } 17 connect(dummy.next); 18 } 19 }
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