LeetCode – Combinations (Java)

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],
]

Java Solution

public ArrayList<ArrayList<Integer>> combine(int n, int k) {
	ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
 
	if (n <= 0 || n < k)
		return result;
 
	ArrayList<Integer> item = new ArrayList<Integer>();
	dfs(n, k, 1, item, result); // because it need to begin from 1
 
	return result;
}
 
private void dfs(int n, int k, int start, ArrayList<Integer> item,
		ArrayList<ArrayList<Integer>> res) {
	if (item.size() == k) {
		res.add(new ArrayList<Integer>(item));
		return;
	}
 
	for (int i = start; i <= n; i++) {
		item.add(i);
		dfs(n, k, i + 1, item, res);
		item.remove(item.size() - 1);
	}
}
Category >> Algorithms >> Interview  
If you want someone to read your code, please put the code inside <pre><code> and </code></pre> tags. For example:
<pre><code> 
String foo = "bar";
</code></pre>