#Leetcode# 77. Combinations

https://leetcode.com/problems/combinations/

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

Example:

Input: n = 4, k = 2
Output:
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

代码：

class Solution {
public:
    vector<vector<int>> combine(int n, int k) {
        vector<int> nums;
        for(int i = 1; i <= n; i ++)
            nums.push_back(i);
        set<vector<int>> ans;
        vector<int> v;
        vector<int> out;
        int cnt;
        for(int i = 0; i < (1 << n); i ++) {
            v.clear();
            out.clear();
            cnt = 0;
            A(i, v);
            for(int j = 0; j < v.size(); j ++) 
                if(v[j]) cnt ++;
            
            if(cnt == k) {
                for(int j = 0; j < v.size(); j ++) 
                    if(v[j])
                        out.push_back(nums[j]);
                
                //sort(out.begin(), out.end());
                ans.insert(out);
            }
        }
        return vector<vector<int>>(ans.begin(), ans.end());
    }
    void A(int x, vector<int> &v) {
        while(x) {
            v.push_back(x % 2);
            x /= 2;
        }
    }
};

　　看 FH 的链表看的有点绕 写一会 $Leetcode$ 冷静冷静