1. Description

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

2. Example

If n = 4 and k = 2, a solution is:
[
[2,4],
[3,4],
[2,3],
[1,2],
[1,3],
[1,4],
]

3. Code

import java.util.ArrayList;
import java.util.List;

public class LeetCode0077 {
    public List<List<Integer>> combine(int n, int k) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        if (n == 0 || k == 0 || n < k) {
            return result;
        }
        dfs(1, n, k, result, new ArrayList<Integer>());
        return result;
    }

    private void dfs(int start, int n, int k, List<List<Integer>> result, ArrayList<Integer> current) {
        if (k == 0) {
            result.add(new ArrayList<Integer>(current));
            return;
        }

        if (start > n) {
            return;
        }

        current.add(start);
        dfs(start + 1, n, k - 1, result, current);
        current.remove(current.size() - 1);
        dfs(start + 1, n, k, result, current);
    }

    public static void main(String[] args) {
        LeetCode0077 leetcode = new LeetCode0077();
        List<List<Integer>> result = leetcode.combine(4, 2);
        for (int i = 0; i < result.size(); i++) {
            for (int j = 0; j < result.get(i).size(); j++) {
                System.out.print(result.get(i).get(j) + " ");
            }
            System.out.println();
        }
    }
}

import java.util.ArrayList;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.List;
import java.util.Set;

public class LeetCode0077 {
    public List<List<Integer>> combine(int n, int k) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        if (n == 0 || k == 0 || n < k) {
            return result;
        }
        Set<Integer> set = new HashSet<Integer>();
        for (int i = 1; i <= n; i++) {
            set.add(i);
        }
        dfs(n, k, result, new LinkedList<Integer>(), set);
        return result;

    }

    private void dfs(int n, int k, List<List<Integer>> result, LinkedList<Integer> current, Set<Integer> set) {

        if (current.size() == k) {
            result.add(new LinkedList<Integer>(current));
            return;
        }

        for (int i = current.isEmpty() ? 1 : current.getLast(); i <= n; i++) {
            if (set.contains(i)) {
                current.add(i);
                set.remove(i);
                dfs(n, k, result, current, set);
                current.removeLast();
                set.add(i);
            }
        }
    }

    public static void main(String[] args) {
        LeetCode0077 leetcode = new LeetCode0077();
        List<List<Integer>> result = leetcode.combine(4, 2);
        for (int i = 0; i < result.size(); i++) {
            for (int j = 0; j < result.get(i).size(); j++) {
                System.out.print(result.get(i).get(j) + " ");
            }
            System.out.println();
        }
    }
}