A145.单词接龙

#include <bits/stdc++.h>
using namespace std;
vector<string> words;	 
int visited[25];
string head;	
int n;	
int ans;
bool modify(string& head, string& tail, string& extra) {
    int i;
    int hsize = strlen(head.c_str());
    int tsize = strlen(tail.c_str());
    for (i = 1; i < min(hsize, tsize); i++) {
        if (head.substr(hsize - i, hsize) == tail.substr(0, i)) {
            break;
        }
    }
    if (i == min(hsize, tsize)) {
        return false;
    } else {
        extra = tail.substr(i, tsize);
        head = head + extra;
        return true;
    }
}
void unmodify(string& head, string& tail, string& extra) {
    head = head.substr(0, head.length() - extra.length());
}
void dfs(string head) {
    for (int i = 0; i < n; i++) {
        if (visited[i] < 2) {			
            string tail = words[i];
            if (head.length() == 1) {	
                if (tail[0] == head[0]) {
                    head = tail;
                    if (head.length() >= ans) {
                        ans = head.length();
                    }
                    visited[i]++;
                    dfs(head);
                    visited[i]--;
                    head = tail[0];
                }

            } else if (head.length() > 1) {
                string extra = "";
                if (modify(head, tail, extra)) {
                    if (head.length() >= ans) {
                        ans = head.length();
                    }
                    visited[i]++;
                    dfs(head);
                    visited[i]--;
                    unmodify(head, tail, extra);
                }
            }
        }
    }
}
int main() {
    cin >> n;
    for (int i = 0; i < n; i++) {
        string word;
        cin >> word;
        words.push_back(word);
    }
    cin >> head;
    dfs(head);
    cout << ans;
    return 0;
}