Challenge: Fewest Roads

Lab C++ Intermediate 20 points

Read a graph, then start goal. Print the fewest roads from start to goal using BFS, or -1.

Input

Line 1: n m – places numbered 0 to n − 1 and m roads. Then m lines a b: a two-way road between a and b.

Last line: start goal.

Output

One number.

Example

Input

Output

Sample input available Sample output available

Editor

C++

Hints

dist = -1 everywhere except the start.

dist[w] = dist[v] + 1 the first time you reach w.

Solution explanation

BFS reaches every place first along a path with the fewest roads.

Show solution

#include <iostream>
#include <queue>
#include <vector>
using namespace std;

int main() {
    int n, m;
    cin >> n >> m;
    vector<vector<int>> g(n);
    for (int i = 0; i < m; i++) {
        int a, b;
        cin >> a >> b;
        g[a].push_back(b);
        g[b].push_back(a);
    }
    int s, t;
    cin >> s >> t;
    vector<int> dist(n, -1);
    queue<int> q;
    dist[s] = 0;
    q.push(s);
    while (!q.empty()) {
        int v = q.front();
        q.pop();
        for (int w : g[v])
            if (dist[w] == -1) { dist[w] = dist[v] + 1; q.push(w); }
    }
    cout << dist[t] << endl;
    return 0;
}

Output

Answer