【LeetCode】101. Symmetric Tree

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

Note:

Bonus points if you could solve it both recursively and iteratively.

• Difficulty: Easy

题目分析:

这道题目的递归解答比较好想，看图写码即可。

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