[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();}}}