[LeetCode] 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],
]

思路：很简单的递归问题。

vector<vector<int> > combine(int n, int k) {
        int pos = 1;
        vector<int> comb;
        vector<vector<int>> combs;
        combineUtil(n, k, pos, comb, combs );

        return combs;
}

void combineUtil(int n, int k, int pos, vector<int> comb, vector<vector<int>>& combs )
{
// n: 1,2,..., n
// k: select k numbers from [1,n]
// pos: the number from [1,n] that may be selected. 
//      This is the depth of the state stack.
// comb: the selected numbers will be stored in comb.
// combs: a completed combination will be stored in combs.

        if(comb.size()==k)
        {
                combs.push_back(comb);
                return;
        }

        if( n-pos+1<k-comb.size() )
                return;

        comb.push_back(pos);
        pos++;
        combineUtil(n, k, pos, comb, combs );
        comb.pop_back();
        combineUtil(n, k, pos, comb, combs );
}