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;
         }

     }
 };