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], ]
Backtracking
class Solution { vector<vector<int> > m_res; vector<int> m_array; public: void dfs(int n, int k, int start) { if(k == m_array.size()) { m_res.push_back(m_array); return; } for(int i = start; i <= n; i++) { m_array.push_back(i); dfs(n, k, i + 1); m_array.pop_back(); } } vector<vector<int> > combine(int n, int k) { dfs(n, k, 1); return m_res; } };
原文：http://www.cnblogs.com/diegodu/p/4335129.html