1. 题目

给定一个二叉树，检查它是否是镜像对称的。

例如，二叉树 [1,2,2,3,4,4,3] 是对称的。1/ \2   2/ \ / \
3  4 4  3
但是下面这个 [1,2,2,null,3,null,3] 则不是镜像对称的:1/ \2   2\   \3    3

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/symmetric-tree
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

2. 解题

2.1 递归

class Solution {
public:bool isSymmetric(TreeNode* root) {return sym(root,root);}bool sym(TreeNode *t1, TreeNode *t2){if((!t1&&t2)||(t1&&!t2))return false;else if(!t1&&!t2)return true;elsereturn(t1->val==t2->val && sym(t1->left,t2->right)&& sym(t1->right,t2->left));}
};

2.2 循环

class Solution {
public:bool isSymmetric(TreeNode* root) {TreeNode *t1, *t2;queue<TreeNode*> q;q.push(root);q.push(root);while(!q.empty()){t1 = q.front();q.pop();t2 = q.front();q.pop();if((!t1&&t2)||(t1&&!t2))return false;else if(!t1&&!t2)continue;else//t1,t2都存在{if(t1->val != t2->val)return false;q.push(t1->left);q.push(t2->right);q.push(t1->right);q.push(t2->left);}}return true;}
};

《剑指Offer》同题：面试题28. 对称的二叉树

递归

class Solution {bool ans = true;
public:bool isSymmetric(TreeNode* root) {dfs(root,root);return ans;}void dfs(TreeNode* r1, TreeNode* r2){if(!ans || (!r1 && !r2))return;if((!r1 && r2)||(r1 && !r2)){ans = false;return;}if(r1->val != r2->val)ans = false;dfs(r1->left,r2->right);dfs(r1->right,r2->left);}
};

循环

class Solution {
public:bool isSymmetric(TreeNode* root) {queue<TreeNode*> q;q.push(root);q.push(root);TreeNode *r1, *r2;while(!q.empty()){r1 = q.front();	q.pop();r2 = q.front();	q.pop();if((!r1 && r2)||(r1 && !r2))return false;if(r1 && r2){if(r1->val != r2->val)return false;q.push(r1->left);q.push(r2->right);q.push(r1->right);q.push(r2->left);}}return true;}
};