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);
         }
     }
 }