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

还是很简单的，递归方法如下：

/**
 * 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;
        }
        TreeNode left = root.left;
        TreeNode right = root.right;
        return querySymmetric(left,right);
    }
    public boolean querySymmetric(TreeNode left, TreeNode right){
        if(left == null && right == null){
            return true;
        }
        if(left == null && right != null){
            return false;    
        }
        if(left != null && right == null){
            return false;
        }
        if(left.val == right.val){
            if(!querySymmetric(left.left,right.right)){
                return false;
            }
            if(!querySymmetric(left.right,right.left)){
                return false;
            }
            return true;
        }else{
            return false;
        }
    }
}