[leetcode] Combinations

From : https://leetcode.com/problems/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],
]

class Solution {
public:
    void putRest(vector<vector<int>>& res, vector<int>& cur, int lastIndex, int needNum, int n) {
        if(needNum == 0) {res.push_back(cur); return;}
        for(int i=lastIndex+1; i<=n-needNum+1; i++) {
            cur.push_back(i);
            putRest(res, cur, i, needNum-1, n);
            cur.pop_back();
        }
    }
    
    vector<vector<int>> combine(int n, int k) {
        vector<vector<int>> res;
        if(k>n || k<=0) return res;
        int last = n-k+1;
        for(int i=1; i<=last; i++) {
            vector<int> cur;
            cur.push_back(i);
            putRest(res, cur, i, k-1, n);
        }
        return res;
    }
};

public class Solution {
	public List<List<Integer>> combine(int n, int k) {
		List<List<Integer>> ans = new ArrayList<List<Integer>>();
		if (n >= k && k > 0) {
			combine(1, k, n, new LinkedList<Integer>(), ans);
		}
		return ans;
	}

	private void combine(int I, int K, int N, LinkedList<Integer> cur,
			List<List<Integer>> ans) {
		if (K == 0) {
			ans.add(new ArrayList<Integer>(cur));
			return;
		}
		for (int i = I; i <= N - K + 1; ++i) {
			cur.add(i);
			combine(i + 1, K - 1, N, cur, ans);
			cur.removeLast();
		}
	}
}