Leetcode #101 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 a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    bool symmetric(TreeNode *p, TreeNode *q)    {    if(!p && !q) return true;    if(p && !q || !p && q) return false;    if(p->val != q->val) return false;    bool b1 = symmetric(p->left , q->right);    bool b2 = symmetric(p->right , q->left);    return b1 && b2;    }        bool isSymmetric(TreeNode* root) {        if(!root)     return true;    return symmetric(root,root);    }};