101.symmetric-tree.cpp

/*
 * @lc app=leetcode id=101 lang=cpp
 *
 * [101] Symmetric Tree
 *
 * https://leetcode.com/problems/symmetric-tree/description/
 *
 * algorithms
 * Easy (47.78%)
 * Likes:    5741
 * Dislikes: 155
 * Total Accepted:    841.7K
 * Total Submissions: 1.7M
 * Testcase Example:  '[1,2,2,3,4,4,3]'
 *
 * Given the root of a binary tree, check whether it is a mirror of itself
 * (i.e., symmetric around its center).
 * 
 * 
 * Example 1:
 * 
 * 
 * Input: root = [1,2,2,3,4,4,3]
 * Output: true
 * 
 * 
 * Example 2:
 * 
 * 
 * Input: root = [1,2,2,null,3,null,3]
 * Output: false
 * 
 * 
 * 
 * Constraints:
 * 
 * 
 * The number of nodes in the tree is in the range [1, 1000].
 * -100 <= Node.val <= 100
 * 
 * 
 * 
 * Follow up: Could you solve it both recursively and iteratively?
 */

// @lc code=start
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    bool isSymmetric1(TreeNode* root) {
        return core(root->left, root->right);
    }
    bool core(TreeNode* left, TreeNode* right) {
        if (left == nullptr && right == nullptr)
            return true;
        if (left == nullptr || right == nullptr)
            return false;
        
        return left->val == right->val && core(left->left, right->right) && core(left->right, right->left);
    }
    bool isSymmetric(TreeNode* root) {
        if (root == nullptr)
            return true;
        queue<TreeNode*> que;
        que.push(root->left);
        que.push(root->right);
        while (!que.empty()) {
            TreeNode* left = que.front();
            que.pop();
            TreeNode* right = que.front();
            que.pop();
            if (left == nullptr && right == nullptr)
                continue;
            if (left == nullptr || right == nullptr)
                return false;
            if (left->val != right->val)
                return false;
            que.push(left->left);
            que.push(right->right);
            que.push(left->right);
            que.push(right->left);
        }
        return true;
    }
};
// @lc code=end