[leetcode]101. Symmetric Tree对称树

发布时间：2020-12-14 04:18:02 所属栏目：大数据来源：网络整理

导读：Given a binary tree,check whether it is a mirror of itself (ie,symmetric around its center). For example,this binary tree? [1,2,3,4,3] ?is symmetric: 1 / 2 2 / / 3 4 4 3 ? But the following? [1,null,3] ?is not: 1 / 2 2 3 3 ? Note: Bonu

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

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

    1
   /   2   2
 /  / 3  4 4  3

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

    1
   /   2   2
         3    3

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

题目

给定二叉树，判断其是否左右对称

思路

DFS

代码

 1 /* 
 2 Time:O(n)，
 3 Space: O(logn) 即 O(h) 树的deepest hight
 4 */
 5 public class Solution {
 6     public boolean isSymmetric(TreeNode root) {
 7         if (root == null) return true;
 8         return isSymmetric(root.left,root.right);
 9     }
10     private static boolean isSymmetric(TreeNode p,TreeNode q) {
11         if (p == null && q == null) return true;   // 终止条件
12         if (p == null || q == null) return false;  // 终止条件
13         return p.val == q.val      // 三方合并
14                 && isSymmetric(p.left,q.right)
15                 && isSymmetric(p.right,q.left);
16     }
17 }

（编辑：李大同）

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