LeetCode（1） Symmetric Tree

从简单的道题目開始刷题目：

Symmetric Tree

题目：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

题目分析：

第一道题目简单的题目，主要利用递归方法，保存左右两个结点。对于LeftNode结点和RightNode结点，推断LeftNode的左结点和RightNode的右结点和LeftNode的右结点和RightNode的左结点是否相等就可以，仅仅要有不相等就能够结束。跳出递归。

代码：

/**
 * 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 check(TreeNode *leftNode,TreeNode *rightNode)
    {
        if(leftNode == NULL && rightNode == NULL)
            return true;
        if(leftNode == NULL ||rightNode ==NULL)
            return false;
        if(leftNode ->val != rightNode->val)
            return false;

        return  check(leftNode->left,rightNode->right) && check(leftNode->right,rightNode->left);

    }

    bool isSymmetric(TreeNode *root) {

        if(root == NULL)
        return true;
        return check(root->left,root->right);
    }
};