Problem

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

Example

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

Code

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
static int var = [](){
    std::ios::sync_with_stdio(false);
    cin.tie(NULL);
    return 0;
}();
class Solution {
public:
    bool isSymmetric(TreeNode* root) {
        if(root==NULL)
            return true;
        else
            return Symmetric(root->left,root->right);
    }
    bool Symmetric(TreeNode* left,TreeNode* right){
        if(left==NULL || right==NULL){
            return left==right;
        }
        else{
            return (left->val==right->val) && Symmetric(left->left,right->right) && Symmetric(left->right,right->left);
        }
    }
};

Result

101. Symmetric Tree.png