[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);
    }
};