77. Combinations

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],
]
  • Easy
public List<List<Integer>> combine(int n, int k) {
     List<List<Integer>> result = new ArrayList<>();
     backtrack(n, k, new ArrayList<Integer>(), result, 1);
     return result;
 }

 public void backtrack(int n, int k, ArrayList<Integer> temp, List<List<Integer>> result, int start) {
     if(temp.size() == k) {
         result.add(new ArrayList<>(temp));
         return;
     }
     for(int i = start; i <= n; i++) {
         temp.add(i);
         backtrack(n, k, temp, result, i+1);
         temp.remove(temp.size()-1);
     }
 }