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

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isSymmetric(TreeNode *root) {
        if (root == NULL) return true;
        return isMirror(root->left, root->right);
    }
   
    bool isMirror(TreeNode* t1, TreeNode* t2) {
        if (t1 == NULL && t2 == NULL) return true;
        if (t1 == NULL || t2 == NULL || t1->val != t2->val) return false;
        return isMirror(t1->left, t2->right) && isMirror(t1->right, t2->left);
    }
  
};