77. Combinations(回溯)

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],
]

class Solution {
    List<List<Integer>> res = new ArrayList<>();
    public List<List<Integer>> combine(int n, int k) {
        List<Integer> temp = new ArrayList<Integer>();
        help(temp,n,k,1);
        return res;
            
    }
    private void help(List<Integer> temp,int n ,int k ,int index){
        if(k==0){
            res.add(new ArrayList<Integer>(temp));
        }
        for(int i = index;i<=n;i++){
            temp.add(i);
            help(temp,n,k-1,i+1);
            temp.remove(temp.size()-1);
        }
    }
}