leetcode 216 组合总和III

package com.example.lettcode.dailyexercises;

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

/**
 * @Class CombinationSum3
 * @Description 216 组合总和III
 * 找出所有相加之和为 n 的 k 个数的组合。组合中只允许含有 1 - 9 的正整数，并且每种组合中不存在重复的数字。
 * <p>
 * 说明：
 * 所有数字都是正整数。
 * 解集不能包含重复的组合。
 * <p>
 * 示例 1:
 * 输入: k = 3, n = 7
 * 输出: [[1,2,4]]
 * <p>
 * 示例 2:
 * 输入: k = 3, n = 9
 * 输出: [[1,2,6], [1,3,5], [2,3,4]]
 * @Author
 * @Date 2020/9/11
 **/
public class CombinationSum3 {
    public static List<List<Integer>> combinationSum3(int k, int n) {
        if (k <= 0 || n <= 0) {
            return new ArrayList<>();
        }
        List<List<Integer>> res = new ArrayList<>();
        List<Integer> integerList = new ArrayList<>();
        int[] combines = new int[]{1, 2, 3, 4, 5, 6, 7, 8, 9};
        recur(res, integerList, combines, k, n, 0);
        return res;
    }

    // 回溯
    public static void recur(List<List<Integer>> res, List<Integer> integers, int[] combines, int k, int n, int idx) {
        // 判断找到的一组列表是否符合要求
        if (n == 0 && integers.size() == k) {
            res.add(new ArrayList<>(integers));
            return;
        }
        if (idx >= combines.length || integers.size() > k) {
            return;
        }
        // 包含当前位置的元素
        if (n - combines[idx] >= 0) {
            integers.add(combines[idx]);
            recur(res, integers, combines, k, n - combines[idx], idx + 1);
            integers.remove(integers.size() - 1);
        }
        // 跳过当前位置的元素
        recur(res, integers, combines, k, n, idx + 1);
    }

    public static void main(String[] args) {
        int k = 3;
        int n = 7;
        List<List<Integer>> res = combinationSum3(k, n);
        System.out.println("combinationSum3 demo01 result:");
        for (List<Integer> integerList : res) {
            for (Integer integer : integerList) {
                System.out.print("," + integer);
            }
            System.out.println();
        }

        k = 3;
        n = 9;
        res = combinationSum3(k, n);
        System.out.println("CombinationSum3 demo02 result:");
        for (List<Integer> integerList : res) {
            for (Integer integer : integerList) {
                System.out.print("," + integer);
            }
            System.out.println();
        }

        k = 3;
        n = 15;
        res = combinationSum3(k, n);
        System.out.println("CombinationSum3 demo03 result:");
        for (List<Integer> integerList : res) {
            for (Integer integer : integerList) {
                System.out.print("," + integer);
            }
            System.out.println();
        }
    }
}