LeetCode：101、对称二叉树

 递归法：

/*** Definition for a binary tree node.* public class TreeNode {*     int val;*     TreeNode left;*     TreeNode right;*     TreeNode() {}*     TreeNode(int val) { this.val = val; }*     TreeNode(int val, TreeNode left, TreeNode right) {*         this.val = val;*         this.left = left;*         this.right = right;*     }* }*/
class Solution {public boolean isSymmetric(TreeNode root) {if(root==null)return true;return compare(root.left,root.right);}public boolean compare(TreeNode left,TreeNode right){if(left==null&&right!=null)return false;//左空右不空，非对称else if(left!=null&&right==null)return false;//右空左不空，非对称else if(left==null&&right==null)return true;//左空右空，对称else if(left.val!=right.val)return false;//左值≠右值，非对称boolean outside=compare(left.left,right.right);boolean inside=compare(left.right,right.left);boolean isSame=outside&&inside;return isSame;}
}