LeetCode Combinations
Given two integers n and k, return all possible combinations of k numbers out of 1,2,...,n.
import java.util.ArrayList;
import java.util.List;
class Solution {
public List<List<Integer>> combine(int n, int k) {
return combine(1, n + 1, k);
}
public List<List<Integer>> combine(int low, int upper, int k) {
List<List<Integer>> result = new ArrayList<List<Integer>>();
if (k == 1) {
for (int i = low; i < upper; i++) {
List<Integer> r = new ArrayList<Integer>();
r.add(i);
result.add(r);
}
return result;
}
for (int i = low; i < upper; i++) {
List<List<Integer>> r = combine(i + 1, upper, k - 1);
for (List<Integer> a : r) {
a.add(0, i);
}
result.addAll(r);
}
return result;
}
public static void main(String[] args) {
Solution s = new Solution();
List<List<Integer>> lists = s.combine(4, 2);
for(List res :lists){
System.out.println(res);
}
}
};
题目二:给定一个数组arr[]和一个整数k,在数组中选择元素组成一个长度为K的数组,打印最终的子数组
Example:arr={6,34,5,65}, K=2;
最终子数组:{6,34},{6,5},{6,65},{34,5},{34,65},{5,65}
参考代码:
import java.util.ArrayList;
import java.util.List;
public class Main {
public static void main(String[] args) {
int[] arr = {6, 34, 5, 65};
int k = 5;
// List<List<Integer>> lists = combine(1, 5, 2);
List<List<Integer>> lists = combine2(arr, 0, arr.length, 2);
for (List res : lists) {
System.out.println(res);
}
}
public static List<List<Integer>> combine2(int[] arr, int low, int upper, int k) {
List<List<Integer>> res = new ArrayList<List<Integer>>();
if (k == 1) {
for (int i = low; i < upper; i++) {
List<Integer> list = new ArrayList<>();
list.add(arr[i]);
res.add(list);
}
return res;
}
for (int i = low; i < upper; i++) {
List<List<Integer>> r = combine2(arr, i + 1, upper, k - 1);
for (List<Integer> l : r) {
l.add(0, arr[i]);
}
res.addAll(r);
}
return res;
}
}