77. Combinations

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],
]

Solution:

Use DFS to traverse all permutations.

When we find the candidate list has length of k, we put it into the result and stop adding new elements.



Code:

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