给出每个站点之间的最短距离，求出最短路径，用unordered_map来实现，让你实现find_cheapest_transform函数

#include <map>#include <set>#include <list>#include <cmath>#include <ctime>#include <deque>#include <queue>#include <stack>#include <string>#include <bitset>#include <cstdio>#include <limits>#include <vector>#include <climits>#include <cstring>#include <cstdlib>#include <fstream>#include <numeric>#include <sstream>#include <iostream>#include <algorithm>#include <unordered_map>using namespace std;/** Complete the function below.*/int find_cheapest_transform(const string &input, const string &output,  unordered_map<char, unordered_map<char, int> > cost_function){    int sum = 0;    int len = input.size();    int distance[len][len];    auto row = cost_function.begin();    for(int i = 0;i < len;i++,row++)    {        auto col = row->second.begin();        for(int j = 0;j < len;j++,col++)        {            distance[i][j] = col->second;            cout<<distance[i][j]<<" ";        }        cout<<endl;    }    for(int i = 0;i < len;i++)    {        for(int j = 0;j < len;j++)        {            for(int k = 0;k < len;k++)            {                if(distance[j][i]+distance[i][k] < distance[j][k])                    distance[j][k] = distance[j][i]+distance[i][k];            }        }    }    row = cost_function.begin();    for(int i = 0;i < len;i++,row++)    {        auto col = row->second.begin();        for(int j = 0;j < len;j++,col++)        {            col->second = distance[i][j];        }    }    for (int i = 0; i < len; i++) {        char s = input[i];        char d = output[i];         sum += cost_function.at(s).at(d);     }    return sum;}int main() {    int res;    string _input = "abcd";    string _output = "bcda";    int rows = 4;    int cols = 4;    unordered_map<char, unordered_map<char, int> > map;    unordered_map<char, int> s;    s['a'] = 0;    s['b'] = 2;    s['c'] = 4;    s['d'] = 5;    map['a'] = s;    unordered_map<char, int> t;    t['a'] = 4;    t['b'] = 0;    t['c'] = 9;    t['d'] = 6;    map['b'] = t;    unordered_map<char, int> m;    m['a'] = 1;    m['b'] = 4;    m['c'] = 0;    m['d'] = 2;    map['c'] = m;    unordered_map<char, int> n;    n['a'] = 3;    n['b'] = 2;    n['c'] = 1;    n['d'] = 0;    map['d'] = n;    char chars[4] = { 'a', 'b', 'c', 'd' };    res = find_cheapest_transform(_input, _output, map);    cout << res << endl;    return 0;}