[Leetcode 63] 77 Combinations

Problem:

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],
]

Analysis:

Simple backtracking problem.

Code:

class Solution {
public:
    vector<vector<int> > res;
    int kk;
    int nn;
    
    vector<vector<int> > combine(int n, int k) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        res.clear();
        vector<int> path;
        kk = k;
        nn = n;
        bc(0, path, 1);
        
        return res;
    }
    
    void bc(int len, vector<int> path, int cur) {
        if (len == kk) {
            res.push_back(path);
            return ;
        }
        
        for (int i=cur; i<=nn; i++) {
            path.push_back(i);
            bc(len+1, path, i+1);
            path.pop_back();
        }
        
        return ;
    }
};