Leetcode 101. Symmetric Tree(easy)
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.
/* 递归实现: 要考虑到左右两个子树去递归,所以重写函数 */ /** * 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 helper(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 -> val == right -> val){ return helper(left -> left, right -> right) && helper(left -> right, right -> left); }else{ return false; } } bool isSymmetric(TreeNode* root) { if (root == NULL){ return true; } return helper(root -> left, root -> right); } };