binaryTreeLevelOrderTraversalII.cpp

// Source: https://leetcode.com/problems/binary-tree-level-order-traversal/
// Author: Miao Zhang
// Date:   2021-01-18

/**
 * 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) {
        vector<vector<int>> res;
        if (root == nullptr) return res;
        queue<TreeNode*> q;
        q.push(root);
        while (!q.empty()) {
            int n = q.size();
            vector<int> tmp;
            for (int i = 0; i < n; i++) {
                TreeNode* node = q.front();
                q.pop();
                tmp.push_back(node->val);
                if (node->left) {
                    q.push(node->left);
                }
                if (node->right) {
                    q.push(node->right);
                }
            }
            res.push_back(tmp);
        }
        reverse(res.begin(), res.end());
        return res;
    }
};