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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

 

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

 

LeetCode 101. Symmetric Tree