【LeetCode】Combinations

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],
]

递推点：加入i后，下一个加入的元素需要遍历i+1~n

因此可以基于k做递归。

base case: k=1，把所有的元素都逐个加入集合。

class Solution {
public:
    vector<vector<int> > combine(int n, int k) {
        vector<vector<int> > result;
        vector<int> cur;
        Helper(result, cur, 1, n, k);
        return result;
    }
    void Helper(vector<vector<int> >& result, vector<int> cur, int begin, int n, int k)
    {
        if(k == 1)
        {
            for(int i = begin; i <= n; i ++)
            {
                cur.push_back(i);
                result.push_back(cur);
                cur.pop_back();
            }
        }
        else
        {
            for(int i = begin; i <= n; i ++)
            {
                cur.push_back(i);
                Helper(result, cur, i+1, n, k-1);
                cur.pop_back();
            }
        }
    }
};