[LeetCode] Combinations

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: Note that combine(n, k) is the union of the following two, depending on whether n is used:
1. combine(n - 1, k).
2. combine(n - 1, k - 1), plus the element n.

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public class Solution { public List<List<Integer>> combine(int n, int k) { ArrayList<List<Integer>> list = new ArrayList<List<Integer>>(); if (n < k || k == 0) { return list; } if (k == 1) { for (int i = 1; i <= n; i++) { ArrayList<Integer> l = new ArrayList<Integer>(); l.add(i); list.add(l); } return list; } list.addAll(combine(n - 1, k)); List<List<Integer>> li = combine(n - 1, k - 1); for (List<Integer> ll : li) { ArrayList<Integer> l = new ArrayList<Integer>(ll); l.add(n); list.add(l); } return list; } }