leetcode 332

问题

Given a list of airline tickets represented by pairs of departure and arrival airports [from, to], reconstruct the itinerary in order. All of the tickets belong to a man who departs from JFK. Thus, the itinerary must begin with JFK.

Note:

  1. If there are multiple valid itineraries, you should return the itinerary that has the smallest lexical order when read as a single string. For example, the itinerary ["JFK", "LGA"] has a smaller lexical order than ["JFK", "LGB"].
  2. All airports are represented by three capital letters (IATA code).
  3. You may assume all tickets form at least one valid itinerary.

Example 1:

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Input: [["MUC", "LHR"], ["JFK", "MUC"], ["SFO", "SJC"], ["LHR", "SFO"]]
Output: ["JFK", "MUC", "LHR", "SFO", "SJC"]

Example 2:

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Input: [["JFK","SFO"],["JFK","ATL"],["SFO","ATL"],["ATL","JFK"],["ATL","SFO"]]
Output: ["JFK","ATL","JFK","SFO","ATL","SFO"]
Explanation: Another possible reconstruction is ["JFK","SFO","ATL","JFK","ATL","SFO"].
             But it is larger in lexical order.

分析

思路1：暴力一点的解法可是使用DFS，直到找到一条把所有机票都用光的路径。

思路2：使用Hierholzer算法求欧拉路径。

代码

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class Solution {
public:
    vector<string> findItinerary(vector<vector<string>>& tickets) {
        unordered_map<string, multiset<string>> mp;
        vector<string> ans;
        for (vector<string> s: tickets) {
            mp[s[0]].insert(s[1]);
        }
        visit("JFK", ans, mp);
        reverse(ans.begin(), ans.end());
        return ans;
    }
    void visit(string city, vector<string> &ans, unordered_map<string, multiset<string>> &mp) {
        while (mp[city].size() != 0) {
            string next = *mp[city].begin();
            mp[city].erase(mp[city].begin());
            visit(next, ans, mp);
        }
        ans.push_back(city);
    }
};