Leetcode 77. 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],
]

s思路：
1. for+recursive：backtracking　或者dfs

class Solution {public:    void helper(vector<vector<int>>&res,vector<int>&cur,int k,int n,int num){        if(k==0){res.push_back(cur);return;}        for(int i=num;i<=n-k+1;i++){            cur.push_back(i);            helper(res,cur,k-1,n,i+1);            cur.pop_back();         }    }    vector<vector<int>> combine(int n, int k) {        vector<vector<int>> res;        vector<int> cur;        helper(res,cur,k,n,1);        return res;    }};//优化了，不用push_back()和pop_back，改用[]访问。class Solution {public:    void helper(vector<vector<int>>&res,vector<int>&cur,int k,int n,int num,int pos){        //        if(pos==k){res.push_back(cur);return;}        for(int i=num;i<=n;i++){            cur[pos]=i;            helper(res,cur,k,n+1,i+1,pos+1);        }    }    vector<vector<int>> combine(int n, int k) {        //        vector<vector<int>> res;        vector<int> cur(k,0);        helper(res,cur,k,n-k+1,1,0);        return res;    }};