Combinations--Leetcode

Given-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],  [up],  [1,3],  [1,4],]

Basic ideas:

1, using recursion

2, each layer recursion, the use of the loop, the starting value of the loop, increment by layer. such as the first layer, starting from 1, the 2nd floor starts from the 2nd.

3. When a combination is generated. And with this group of cooperation samples, copy one, on this basis, generate the next combination.

Class Solution {public:    vector<vector<int> > Combine (int n, int k) {        vector<vector<int> > ans (1);        Helper (ans, 1, n, k);        Ans.pop_back ();        return ans;    }        void Helper (Vector<vector<int> > &ans, int start, int n, int k) {        if (!k)            return ans.push_back (ans . back ());                    for (int i=start; i<=n; i++) {            ans.back (). push_back (i);            Helper (ans, i+1, N, k-1);            Ans.back (). Pop_back ();}}    ;

Combinations--Leetcode