一个拓扑排序问题

n个人排队， 每个人都有要求Request， 具体每个要求是希望排在某个人之前或者之后， 用类RequestItem表示。

例如有 1， 2， 3三个人， 1希望排在2之后3之前， 2希望排在1之前， 3希望排在1， 2之后。输出一个合理的排列

import java.util.HashMap;import java.util.LinkedList;import java.util.List;class RequestItem{String name;boolean front;}class Request{String name;List<RequestItem> items;}public class FindValidOrder {static List<String> validOrder(List<String> names, List<Request> requests){HashMap<String, List<String>> maps = new HashMap<String, List<String>>();for(String name: names){maps.put(name, new LinkedList<String>());}for(Request re: requests){for(RequestItem item: re.items){if(item.front){List<String> tmp = maps.get(item.name);if(!tmp.contains(re.name)){tmp.add(re.name);}}else{List<String> tmp = maps.get(re.name);if(!tmp.contains(item.name)){tmp.add(item.name);}}}}HashMap<String, Integer> counts = new HashMap<String, Integer>();for(String name: names){counts.put(name, 0);}for(String name: names){List<String> tmp = maps.get(name);for(String s: tmp){int tmpcount = counts.get(s)+1;counts.put(s, tmpcount);}}List<String> queue = new LinkedList<String>();while(!counts.isEmpty()){boolean found = false;String next = null;for(String name: counts.keySet()){if(counts.get(name)==0){next = name;found = true;}}if(!found) break;counts.remove(next);for(String s: maps.get(next)){int tmpcount = counts.get(s)-1;counts.put(s, tmpcount);}queue.add(next);}if(queue.size() == names.size()) return queue;return null;}public static void main(String [] args){List<String> names = new LinkedList<String>();names.add("1");names.add("2");names.add("3");List<Request> requests = new LinkedList<Request>();Request r = new Request();r.name = "1";r.items = new LinkedList<RequestItem>();RequestItem item = new RequestItem();item.name = "2";item.front = true;r.items.add(item);RequestItem item2 = new RequestItem();item2.name = "3";item2.front = false;r.items.add(item2);requests.add(r);Request r2 = new Request();r2.name = "2";r2.items = new LinkedList<RequestItem>();RequestItem item3 = new RequestItem();item3.name = "1";item3.front = false;r2.items.add(item3);requests.add(r);Request r3 = new Request();r3.name = "3";r3.items = new LinkedList<RequestItem>();RequestItem item4 = new RequestItem();item4.name = "2";item4.front = true;r3.items.add(item4);RequestItem item5 = new RequestItem();item5.name = "1";item5.front = true;r3.items.add(item5);requests.add(r3);List<String> order = validOrder(names, requests);System.out.println(order);}}