LeetCode101. Symmetric Tree

依旧是尝试DFS解题，这次选择了LeetCode101 Symmetric Tree

题意如下：

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

判断二叉树是否对称，直接用递归法进行深度搜索即可：

/** * 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;        else            return Symmetric(root->left, root->right);    }     bool Symmetric(TreeNode* left, TreeNode* right) {        if (left == NULL || right == NULL)            return left == right;        else if (left->val == right->val)            return Symmetric(left->left, right->right) && Symmetric(left->right, right->left);        else            return false;    }};