ABC有多少种排列组合

package com.thinkgem.jeesite.test;

import com.google.common.collect.Lists;

import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class PermutationsTest {

    public static void main(String[] args) {
        System.out.println(getSize(4));
        System.out.println(getDeduplicationList(Lists.newArrayList("A", "B", "D", "D")));
        System.out.println(getList(Lists.newArrayList("A", "B", "D", "D")));
    }

    /**
     * 去除重复的数据
     * @param list
     * @return
     */
    public static List<List<String>> getDeduplicationList(List<String> list) {
        List<List<String>> listLists = getList(list);
        Map<String, List<String>> stringListMap = listLists.parallelStream().collect(Collectors.toMap(keys -> {
            StringBuffer sb = new StringBuffer();
            keys.forEach(key -> {
                sb.append(key);
            });
            return sb.toString();
        }, value -> value, (v1, v2) -> v1));
        return Lists.newArrayList(stringListMap.values());
    }

    /**
     * 不进行去重，也就是list中每个字符都不相同
     *
     * @param list
     * @return
     */
    public static List<List<String>> getList(List<String> list) {
        List<List<String>> listLists = Lists.newArrayList();
        for (int i = 0; i < list.size(); i++) {
            listLists = getSpecies(i + 1, listLists, list.get(i));
        }
        return listLists;
    }

    /**
     * 实现方式为前一次获取的组合List，取出前一次组合的每一个list信息，新增的值可以插入list索引位置0-len位，
     * 其余的值判断索引位置是否小于它占用的位置，小于不用管，大于的话，所有的位置都往后移一位
     *
     * @param len
     * @param listLists
     * @param value
     * @return
     */
    private static List<List<String>> getSpecies(int len, List<List<String>> listLists, String value) {
        if (len == 1) {
            List<String> listList = Lists.newArrayList(value);
            listLists.add(listList);
            return listLists;
        }
        List<List<String>> valueLists = Lists.newArrayList();
        for (List<String> listList : listLists) {
            for (int i = 0, size = listList.size() + 1; i < size; i++) {
                String[] strings = new String[size];
                strings[i] = value;
                for (int j = 0; j < listList.size(); j++) {
                    if (j < i) {
                        strings[j] = listList.get(j);
                    } else {
                        strings[j + 1] = listList.get(j);
                    }
                }
                valueLists.add(Lists.newArrayList(strings));
            }
        }
        return valueLists;
    }

    /**
     * 获取数量 多一个参数等于在获取前一次list的基础上把单个list提出，
     * 当前值可以放在list的索引最大位置+1处，表现公式为：f(n)= n * f(n-1)
     *
     * @param len
     * @return
     */
    private static int getSize(int len) {
        if (len == 1) {
            return 1;
        }
        return len * getSize(len - 1);
    }
}