Easy 21 Symmetric Tree（101）

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

For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

1
/ \
2 2
/ \ / \
3 4 4 3
But the following [1,2,2,null,3,null,3] is not:
1
/ \
2 2
\ \
3 3

Solution

/** * 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 isSymmetric(TreeNode* root) {      if(root==nullptr) return true;      return Symmetric(root->left,root->right);    }    bool Symmetric(TreeNode *p,TreeNode *q){        if(!q||!p) return !q&&!p;        return q->val==p->val&&Symmetric(p->left,q->right)&&Symmetric(p->right,q->left);    }};