0000 - 0099
0077 - Combinations (Medium)
Problem Link
https://leetcode.com/problems/combinations/
Problem Statement
Given two integers n and k, return all possible combinations of k numbers chosen from the range [1, n].
You may return the answer in any order.
Example 1:
Input: n = 4, k = 2
Output: [[1,2],[1,3],[1,4],[2,3],[2,4],[3,4]]
Explanation: There are 4 choose 2 = 6 total combinations.
Note that combinations are unordered, i.e., [1,2] and [2,1] are considered to be the same combination.Example 2:
Input: n = 1, k = 1
Output: [[1]]
Explanation: There is 1 choose 1 = 1 total combination.Constraints:
1 <= n <= 201 <= k <= n
Approach 1: Backtracking
This question can be solved by standard backtracking. Check out Backtracking section for the detailed explanation.
class Solution {
public:
vector<vector<int>> combine(int n, int k) {
vector<int> chosen;
vector<vector<int>> ans;
function<void(int)> backtrack = [&](int start) {
if (chosen.size() == k) {
ans.push_back(chosen);
return;
}
for (int i = start; i <= n; i++) {
chosen.push_back(i);
backtrack(i + 1);
chosen.pop_back();
}
};
backtrack(1);
return ans;
}
};