下面程序中横线处应该给出的是什么？

#include <iostream>
#include <vector>
#include <map>
#include <algorithm>
using namespace std;

string graphHash(vector<vector<int>>& graph) {
    vector<string> nodeHashes(graph.size());
    for (int i = 0; i < graph.size(); i++) {
        vector<int> neighbors = graph[i];
        sort(neighbors.begin(), neighbors.end());
        string hash;
        for (int neighbor : neighbors) {
            __________________________
        }
        nodeHashes[i] = hash;
    }
    sort(nodeHashes.begin(), nodeHashes.end());
    string finalHash;
    for (string h : nodeHashes) {
        finalHash += h + ";";
    }
    return finalHash;
}