[LeetCode101]Symmetric Tree

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     
13     bool isSymmetric(TreeNode *root) {
14         if (!root) return true;
15         return helper(root->left, root->right);
16     }
17 
18     bool helper(TreeNode* p, TreeNode* q) {
19         if (!p && !q) {
20             return true;
21         } else if (!p || !q) {
22             return false;
23         }
24 
25         if (p->val != q->val) {
26             return false;
27         }
28 
29         return helper(p->left,q->right) && helper(p->right, q->left); 
30     }
31 };