LeetCode —— Combinations

链接：http://leetcode.com/onlinejudge#question_77

原题：

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],]

思路：递归实现

代码：

class Solution {public:    vector<vector<int> > combine(int n, int k) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        vector<int> vec;        vector<vector<int> > collection;        search(n, k, 1, vec, collection);                return collection;    }    private:    void search(int n, int k, int start, vector<int> &vec, vector<vector<int> > &collection) {        if (k == 0) {            collection.push_back(vec);            return;        }                for (int i=start; i<=n-k+1; i++) {            vec.push_back(i);            search(n, k-1, i+1, vec, collection);            vec.pop_back();        }    }};