leetcode-101

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

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 helper(TreeNode left, TreeNode right){
if(left == null || right == null)
{
if(left == right)
return true;
else
return false;
}
if(left.val != right.val)
return false;
else
return helper(left.left,right.right) && helper(left.right,right.left);
}

public boolean isSymmetric(TreeNode root) {

if(root == null)
return true;
else
return helper(root.left,root.right);
}

}