Leetcode 332. Reconstruct Itinerary

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本文介绍了解决LeetCode 332.行程重建问题的两种方法：一种是使用深度优先搜索（DFS）结合回溯和递归；另一种是采用贪心算法。通过这两种方法对比，阐述了如何有效地解决机票行程安排问题。

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Leetcode 332. Reconstruct Itinerary

dfs: 熟悉深度优先搜索的代码结构，回溯+递归，这个版本可以进行优化的地方：其实不需要对链表进行排序，只需要维持一个最大堆的结构就行了。

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.List;
import java.util.PriorityQueue;

public class Solution {
	public static void main(String[] args){
		//String[][] tickets = {{"JFK","SFO"},{"JFK","ATL"},{"SFO","ATL"},{"ATL","JFK"},{"ATL","SFO"}};
		String[][] tickets = {{"JFK","KUL"},{"JFK","NRT"},{"NRT","JFK"}};
		Solution s = new Solution();
		List<String> result = s.findItinerary(tickets);
	}
	
	class SortByString implements Comparator<String> {
		 public int compare(String o1, String o2) {
		  String s1 = (String) o1;
		  String s2 = (String) o2;
		  int  flag = -1;
		   for(int i=0;i<s1.length();i++){
			   if(s1.charAt(i)<s2.charAt(i)){
				   flag = 0;
				   break;
			   }else{
				   flag = 1;
				   break;
			   }
		   }
		   return flag;
		 }
	}
	
    public List<String> findItinerary(String[][] tickets) {
        HashMap<String, ArrayList<String>>  fromto = new HashMap<String,ArrayList<String>>();
        HashMap<String, int[]> used = new HashMap<String,int[]>();
        //HashMap<String, PriorityQueue<String>> ticketsMap = new HashMap<>();
        for(int i=0;i<tickets.length;i++){
        	String[] ticket = tickets[i];
        	if(!fromto.containsKey(ticket[0])){
        		fromto.put(ticket[0], new ArrayList<String>());
        		//used.put(ticket[0], new ArrayList<Integer>());
        	}
        	fromto.get(ticket[0]).add(ticket[1]);
        	//used.get(ticket[0]).add(0);
        }
        
        for(String s : fromto.keySet()){
        	int n = fromto.get(s).size();
        	int[] use = new int[n];
        	used.put(s, use);
        }
        
        //comparator
        Comparator<String> comparator = new Comparator<String>(){
        	   public int compare(String s1, String s2) {
        		   //int min = s1.length()<s2.length()?s1.length():s2.length();
        		   int  flag = -1;
        		   for(int i=0;i<s1.length();i++){
        			   if(s1.charAt(i)<s2.charAt(i)){
        				   flag = 1;
        				   break;
        			   }else{
        				   flag = 0;
        				   break;
        			   }
        		   }
        		   return flag;
        	   }
        };
        	  
        for(String start: fromto.keySet()){
        	ArrayList<String> tmp = fromto.get(start);
        	Collections.sort(tmp);
        	fromto.put(start,tmp);
        	//fromto.put(start, value)
        }
        
        //
        List<String> res = new ArrayList<String>();
        //res.add("JFK");
        List<List<String>> result = new ArrayList<List<String>>();
        dfs("JFK",fromto,used,res,result,tickets.length);
        return result.get(0);
        
    }

    
	private int dfs(String start, HashMap<String, ArrayList<String>> fromto, HashMap<String, int[]> used,
			List<String> res, List<List<String>> result, int len) {
		// TODO Auto-generated method stub
		if(res.size()>len) return 1;
		if(isCompelete(used)){
			res.add(start);
			result.add(new ArrayList<String>(res));
			return 0;
		}else{
				int re = 1;   // 1 stands for fair .
				for(int i=0;i<fromto.get(start).size();i++){
					if(used.get(start)[i]==0){
						used.get(start)[i] = 1;
						String next = fromto.get(start).get(i);
						res.add(start);
						
						if(!isCompelete(used)&&!fromto.containsKey(next)){    // backtrack
							used.get(start)[i] = 0;
							res.remove(res.size()-1);
							continue;
						}
						
						re = dfs(next,fromto,used,res,result,len);
						
						if(re==0){
							break;
						}
						used.get(start)[i] = 0;
						res.remove(res.size()-1);
					}
				}
				return re;
		}
	}


	private boolean isCompelete(HashMap<String, int[]> used) {
		// TODO Auto-generated method stub\
		boolean flag = true;
		for(String s:used.keySet()){
			for(int use :used.get(s)){
				if(use==0){
					flag = false;
				}
			}
		}
		return flag;
	}

	
}

Solution 别人给的Greedy 解法：

 public List<String> findItinerary(String[][] tickets) {
        List<String> ans = new ArrayList<String>();
        if(tickets == null || tickets.length == 0) return ans;
        Map<String, PriorityQueue<String>> ticketsMap = new HashMap<>();
        for(int i = 0; i < tickets.length; i++) {
            if(!ticketsMap.containsKey(tickets[i][0])) ticketsMap.put(tickets[i][0], new PriorityQueue<String>());
            ticketsMap.get(tickets[i][0]).add(tickets[i][1]);
        }

        String curr = "JFK";
        Stack<String> drawBack = new Stack<String>();
        for(int i = 0; i < tickets.length; i++) {
            while(!ticketsMap.containsKey(curr) || ticketsMap.get(curr).isEmpty()) {
                drawBack.push(curr);
                curr = ans.remove(ans.size()-1);
            }
            ans.add(curr);
            curr = ticketsMap.get(curr).poll();
        }
        ans.add(curr);
        while(!drawBack.isEmpty()) ans.add(drawBack.pop());
        return ans;
    }