LeetCode（25）-symmetric tree

题目：

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

For example, this binary tree is symmetric:

1
/ \
2   2
/ \ / \
3  4 4  3
But the following is not:
1
/ \
2   2
\   \
3    3

思路：

首先这是一个求关于中心成对称二叉树的题目

二叉树的思路就是找到一个递归的突破口

首先判断left和right节点的关系来判断，以及(left.left,right.right)以及（left.right,right.left）的关系

代码：

/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public boolean isSymmetric(TreeNode root) {
if(root == null){
return true;
}else{
return isEqual(root.left,root.right);
}
}
public boolean isEqual(TreeNode left,TreeNode right){
if(left == null){
return right == null;
}
if(right == null){
return left == null;
}
if(left.val != right.val){
return false;
}
if(!isEqual(left.left,right.right)){
return false;
}
if(!isEqual(left.right,right.left)){
return false;
}
return true;
}
}