chapter 13

Author: NetherNova

src/chapter13/Main.java

@@ -0,0 +1,46 @@
+package chapter13;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+
+public class Main {
+	
+	public static List<List<Integer>> subsets(List<Integer> list) {
+		if(list.isEmpty()) {
+			List<List<Integer>> ans = new ArrayList<>();
+			ans.add(Collections.emptyList());
+			return ans;
+		}
+		Integer first = list.get(0);
+		List<Integer> rest = list.subList(1, list.size());
+		List<List<Integer>> subans = subsets(rest);
+		List<List<Integer>> subans2 = insertAll(first, subans);
+		return concat(subans, subans2);
+	}
+	
+	public static List<List<Integer>> insertAll(Integer first, List<List<Integer>> subans) {
+		List<List<Integer>> result = new ArrayList<>();
+		for(List<Integer> list : subans) {
+			List<Integer> copyList = new ArrayList<>();
+			copyList.add(first);
+			copyList.addAll(list);
+			result.add(copyList);
+		}
+		return result;
+	}
+	
+	public static List<List<Integer>> concat(List<List<Integer>> first, List<List<Integer>> second) {
+		List<List<Integer>> result = new ArrayList<>(first);
+		result.addAll(second);
+		return result;
+	}
+	
+	public static void main(String[] args) {
+		List<Integer> list = Arrays.asList(new Integer[] {1,2,3,4});
+		System.out.println(subsets(list));
+		
+	}
+
+}