[leetcode.com]算法题目 - 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.

 /**
  * 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 isSymmetric(TreeNode *root) {
         // Start typing your C/C++ solution below
         // DO NOT write int main() function
         if (!root) return true;

         if(!root->left && !root->right)
             return true;

         if(!root->left && root->right)
             return false;

         if(root->left && !root->right)
             return false;

         if(root->left->val == root->right->val)
             return symmetricTree(root->left, root->right);
         else
             return false;
     }

     bool symmetricTree(TreeNode *a, TreeNode *b){
         if(!a && !b)
             return true;

         if((!a && b) || (a && !b))
             return false;

         if(a->val == b->val)
             return symmetricTree(a->left, b->right) && symmetricTree(a->right, b->left);
         else
             return false;
     }
 };

思路：和判断两个tree是否相同有点类似，但是要注意这里判断的是是否对称。好久不写代码，看了别人写的这道题答案，感觉自己写的代码很不标准。不管怎样，先贴出来，慢慢进步吧。