[LeetCode] 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],]

思路：很简单的递归问题。

vector<vector<int> > combine(int n, int k) {        int pos = 1;        vector<int> comb;        vector<vector<int>> combs;        combineUtil(n, k, pos, comb, combs );        return combs;}void combineUtil(int n, int k, int pos, vector<int> comb, vector<vector<int>>& combs ){// n: 1,2,..., n// k: select k numbers from [1,n]// pos: the number from [1,n] that may be selected. //      This is the depth of the state stack.// comb: the selected numbers will be stored in comb.// combs: a completed combination will be stored in combs.        if(comb.size()==k)        {                combs.push_back(comb);                return;        }        if( n-pos+1<k-comb.size() )                return;        comb.push_back(pos);        pos++;        combineUtil(n, k, pos, comb, combs );        comb.pop_back();        combineUtil(n, k, pos, comb, combs );}