[leetcode] combination （不會做）

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],
]

class Solution {
public:
    vector<vector<int> > combine(int n, int k) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        assert(n>=k);
        assert(k>0);
    
        vector<vector<int> >results;
        queue<vector<int> >temp;
        temp.push(vector<int>());
        while(!temp.empty()){
            vector<int> current= temp.front();
            temp.pop();
            int val;
            if (current.size()>0)
                val = current.at(current.size()-1);
            else
                val = 0;
            current.push_back(0);
            for(int i = val+1; i<= n; i++){
                current[current.size()-1]= i;
                if (current.size() == k)
                    results.push_back(current);
                else
                    temp.push(current);
            }
        }
        return results;
    }
};

1. 比DP做要簡單的多，DP的代碼弄了好久也沒弄清楚

2. 注意queue的用法，以及vector中取最後一個值可以用back