Symmetric Tree

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

递归解法：

/**
 * Definition for binary tree
 * 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 isMirror(root.left,root.right);
        }
    }
    public boolean isMirror(TreeNode left,TreeNode right)
    {
        if(left==null||right==null)//注：不能写成left==null&&right==null，报错
        {
            return left==right;
        }
        else
        {
            return (left.val==right.val)&&isMirror(left.left,right.right)&&isMirror(left.right,right.left);
        }
    }
}