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],]

用DFS的思路做，定义好start，end，depth以及max depth。

class Solution {public:    vector<vector<int> > combine(int n, int k) {        vector< vector<int> > result;        if (n < k) {            return result;        }                vector<int> v;        combineUtil(result, v, 1, n, 0, k);        return result;    }        void combineUtil(vector< vector<int> > &result, vector<int> &v, int start,                    int end, int depth, int maxdep) {                if (depth == maxdep) {            result.push_back(v);            return;        }                for (int i = start; i <=end ; i++) {            v.push_back(i);            combineUtil(result, v, i+1, end, depth+1, maxdep);            v.pop_back();        }                            }    };