day15 - 二叉树 part02

zqh2023 / 2023-08-23 / 原文

102. 二叉树的层序遍历

详解

/**
 * 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:
    vector<vector<int>> levelOrder(TreeNode* root) {
        /*
        // 没想到用size来固定每轮取的数量，用了个临时队列做中转
        vector<vector<int>> result;
        queue<TreeNode*> queue_1;
        if(root == NULL)
            return result;
        queue_1.push(root);
        while(!queue_1.empty()){
            vector<int> tmp;
            queue<TreeNode*> queue_2;
            while(!queue_1.empty()){
                TreeNode* node = queue_1.front();
                tmp.push_back(node->val);
                if(node->left != NULL) queue_2.push(node->left);
                if(node->right != NULL) queue_2.push(node->right);
                queue_1.pop();
            }
            while(!queue_2.empty()){
                queue_1.push(queue_2.front());
                queue_2.pop();
            }
            result.push_back(tmp);
        }
        */

        queue<TreeNode*> que;
        if (root != NULL) que.push(root);
        vector<vector<int>> result;
        while (!que.empty()) {
            int size = que.size();
            vector<int> vec;
            // 这里一定要使用固定大小size，不要使用que.size()，因为que.size是不断变化的
            for (int i = 0; i < size; i++) {
                TreeNode* node = que.front();
                que.pop();
                vec.push_back(node->val);
                if (node->left) que.push(node->left);
                if (node->right) que.push(node->right);
            }
            result.push_back(vec);
        }

        return result;
    }
};