leetcode oj Symmetric Tree

Leetcode Question :

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center)

level : easy

/** * 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;        else            return isSymmetric(root->left,root->right);    }    bool isSymmetric(TreeNode *lt,TreeNode *rt) {        if(lt==NULL && rt==NULL)            return true;        else if(lt==NULL && rt!=NULL)            return false;        else if(lt!=NULL && rt==NULL)            return false;        else if(lt!=NULL && rt!=NULL && rt->val !=lt->val )            return false;        else            return isSymmetric(lt->left,rt->right) && isSymmetric(lt->right,rt->left);    }};