【LeetCode从零单刷】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

解答：

最开始非常脑残地希望补全二叉树，然后利用1，2，4，8个节点之间的关系来比较。结果越来越复杂。

还是那句话：碰上树，能递归就递归，不能递归就栈、队列。这次的递归要点：某节点的左子树的左子树 = 右子树的右子树，左子树的右子树 = 右子树的左子树。

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