[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.

Note:子树不需要对称，只需要根书的左右是镜像即可。

1 递归，判断leftNode->left和rightNode->right，leftNode->right和rightNode->left，left->val == right->val

/**
 * 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) {
            if(root == NULL) return true;
            return isSymmetricTree(root->left, root->right);
        }

        bool isSymmetricTree(TreeNode *left, TreeNode *right)
        {
            if(left == NULL && right == NULL) return true; //终止条件
            if(left == NULL || right == NULL) return false;
            if(left->val == right->val &&　　　　　　　　　　//合并
                    isSymmetricTree(left->left, right->right) &&
                    isSymmetricTree(right->left, left->right) 
              )   
                return true;

            return false;
        }

};