仅当集合为空时弹出值背后的直觉

我正在解决一个关于Leetcode的问题: https://leetcode.com/problems/reconstruct-itinerary/description/ . 问题是:

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.

例如,如果 tickets = [["MUC", "LHR"], ["JFK", "MUC"], ["SFO", "SJC"], ["LHR", "SFO"]] ,输出应为: ["JFK", "MUC", "LHR", "SFO", "SJC"] .

我编写了以下代码,这(可以理解)破坏了输入 [["JFK","KUL"],["JFK","NRT"],["NRT","JFK"]] ,因为根据我的代码,节点“NRT”保持未访问状态:

class Solution {
public:
    vector<string> findItinerary(vector<pair<string, string>> tickets) {
        if(tickets.empty()) return vector<string>();

        vector<string> result;
        unordered_map<string, multiset<string>> itinerary;

        for(auto& each : tickets)
            itinerary[each.first].insert(each.second);

        stack<string> myStack;
        myStack.push("JFK");
        while(!myStack.empty()) {
            string topVal=myStack.top();
            result.push_back(topVal);
            myStack.pop();
            if(!itinerary[topVal].empty()) {
                myStack.push(*itinerary[topVal].begin());
                itinerary[topVal].erase(itinerary[topVal].begin());
            }
        }

        return result;
    }
};

为了克服这一问题,其中一个经过投票的解决方案提出了以下小变化:

class Solution {
public:
    vector<string> findItinerary(vector<pair<string, string>> tickets) {
        if(tickets.empty()) return vector<string>();

        vector<string> result;
        unordered_map<string, multiset<string>> itinerary;

        for(auto& each : tickets)
            itinerary[each.first].insert(each.second);

        stack<string> myStack;
        myStack.push("JFK");
        while(!myStack.empty()) {
            string topVal=myStack.top();
            if(itinerary[topVal].empty()) {   //--->this if condition
                result.push_back(topVal);
                myStack.pop();
            }
            else {
                myStack.push(*itinerary[topVal].begin());
                itinerary[topVal].erase(itinerary[topVal].begin());
            }
        }

        reverse(result.begin(), result.end());
        return result;
    }
};

现在,我用这个示例编写了这段代码 [[“JFK”、“KUL”]、[“JFK”、“NRT”]、[“NRT”、“JFK”]] ,并查看它如何将值插入 result 反向矢量;但我不明白 直觉 在该if条件之后:

只有当集合为空时才从堆栈弹出,如何确保该测试用例得到处理?