77. Combinations

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77. Combinations

Description

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:

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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:

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Input: n = 1, k = 1
Output: [[1]]
Explanation: There is 1 choose 1 = 1 total combination.

Constraints:

  • 1 <= n <= 20
  • 1 <= k <= n

Hints/Notes

Solution

Language: C++

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class Solution {
public:
    vector<vector<int>> res;
    vector<int> cur;

    vector<vector<int>> combine(int n, int k) {
        dfs(1, n, k);
        return res;
    }

    void dfs(int index, int n, int k) {
        if (k == 0) {
            res.push_back(cur);
            return;
        }
        // when we are at index, we can pick at most n - index + 1 numbers
        // while we need to pick k numbers
        if (k < n - index + 1) {
            // we can skip this number and go to next directly
            dfs(index + 1, n, k);
        }
        cur.push_back(index);
        dfs(index + 1, n, k - 1);
        cur.pop_back();
    }
};