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

medium

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

Problem

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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

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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

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• 1 <= n <= 20
• 1 <= k <= n

Solution

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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.

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;
  }
};