Combinations
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# LeetCode #77: Combinations (C/C++).

medium

source: https://leetcode.com/problems/combinations/
C/C++ Solution to LeetCode problem 77. Combinations.

## Problem

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.

## Examples

### 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 <= 20
• 1 <= k <= n

## Solution

This one is easily solved using Backtracking

• Becasue there should not be repeated combinations, for recursive call we set the initial index to be one more than the previous call.
• And we add 1 to the number when adding to the combination (because the range is from 1 to n).

For this solution I’m pushing/poping an element to generate the combination, and alternative (a couple of milliseconds faster) is to have a vector of fixed k size and just handle the index of the current element to set.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 class Solution { vector<vector<int>> result; void generateCombinations(int n, int k, int ci, vector<int>& c) { if (c.size() == k) { result.push_back(c); return; } for (int i=ci; i<n; i++) { c.push_back(i+1); generateCombinations(n, k, i+1, c); c.pop_back(); } } public: vector<vector<int>> combine(int n, int k) { result.clear(); vector<int> sol; generateCombinations(n, k, 0, sol); return result; } };