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
Java
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();
}
}
}
Java
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();
}
}
}
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