[Leetcode] Symmetric Tree

递归

/** * 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 isSymmetric_helper(root->right, root->left);    }        bool isSymmetric_helper(TreeNode *p, TreeNode *q){        if(p==NULL&&q==NULL) return true;        if(p==NULL||q==NULL) return false;        if(p->val==q->val){            return isSymmetric_helper(p->left, q->right)&&isSymmetric_helper(p->right, q->left);        }        else return false;    }};