77. Combinations

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

For example,
If n = 4 and k = 2, a solution is:

[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

回溯

vector<vector<int>> result;
vector<int> temp;

vector<vector<int>> combine(int n, int k) {
    combineHelper(n, k, 1);
    return result;
}

void combineHelper(int n, int k, int start){
    if(temp.size() == k){
        result.push_back(temp);
        return;
    }
    for(int i = start; i <= n; ++i){
        temp.push_back(i);
        combineHelper(n, k, i + 1);
        temp.pop_back();
    }
}