【leetcode】77. Combinations【java】

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],

]

public class Solution {
    public List> combine(int n, int k) {
        //回溯法
        List> result = new ArrayList>();
        List tmp = new ArrayList();
        backtrack(result, tmp, n, k, 1);
        return result;
    }
    private void backtrack(List> result, List tmp, int n, int k, int start){
        if (tmp.size() == k){
            result.add(new ArrayList(tmp));
            return;
        }
        for (int i = start; i <= n; i++){
            tmp.add(i);
            backtrack(result, tmp, n, k, i+1);
            tmp.remove(tmp.size() - 1);
        }
    }
}