LeetCode（101）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:

But the following is not:

Note:
Bonus points if you could solve it both recursively and iteratively.

confused what “{1,#,2,3}” means? > read more on how binary tree is serialized on OJ.

分析

判断一棵二叉树是否为对称树；

仍然采用递归的思想，判断该树的左右子树是否对称；

若二叉树p与二叉树q对称，也就是说其根节点相同，p左子树应与q右子树对称，同理，p右子树应与q左子树对称；

AC代码

/** * 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)            return true;        else            //判断左右子树是否对称            return isSymmetricTree(root->left, root->right);    }    bool isSymmetricTree(TreeNode* p, TreeNode* q) {        //如果两个二叉树均为空，则返回true        if (!p && !q)        {            return true;        }        //如果两者其一为空树，则返回false        else if (!p || !q)        {            return false;        }        else{            if (p->val != q->val)                return false;            else                //p左子树应与q右子树对称，同理，p右子树应与q左子树对称                return isSymmetricTree(p->left, q->right) && isSymmetricTree(p->right, q->left);        }    }};