leetcode-101

題目：

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

代碼：

/**
 * 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);
}
}