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;
    }
}