leetcode 101

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.

判断二叉树是否为平衡二叉树。

递归实现。

代码如下：

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