[LeetCode] Symmetric Tree

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

For example, this binary tree is symmetric:

    1

   / \

  2   2

 / \ / \

3  4 4  3

 

But the following is not:

    1

   / \

  2   2

   \   \

   3    3

 

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

解题思路:

递归做很直接。

/**

 * 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 test(TreeNode *a, TreeNode *b)

    {

        if(a == NULL && b == NULL) return true;

        if(a == NULL || b == NULL) return false;

        if(a -> val != b -> val) return false;

        if(test(a -> left, b -> right) && test(a -> right, b -> left)) return true;

        else return false;

    }

    

    bool isSymmetric(TreeNode *root) {

        // IMPORTANT: Please reset any member data you declared, as

        // the same Solution instance will be reused for each test case.

        if(root == NULL) return true;

        return test(root -> left, root -> right);

    }

};

 

 

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