Ladders

#include <bits/stdc++.h>

using namespace std;

int bfs(int x, int z, int n, vector<vector<int>> &adj) {

    queue<int> q, d;
    q.push(x);
    d.push(0);

    vector<bool> visited(n, false);

    while (!q.empty()) {

        int c = q.front();
        q.pop();

        int dist = d.front();
        d.pop();

        if (visited[c])
            continue;

        if (c == z)
            return dist;
        
        for (int y : adj[c]) {
            q.push(y);
            d.push(dist+1);
        }

    }

    return -1;

}

int main() {

    int n;
    cin >> n;

    vector<int> A(n);
    for (int i = 0; i < n; i++)
        cin >> A[i];
    
    if (n == 1) {
        cout << 0 << endl;
        return 0;
    } else if (n == 2) {
        cout << 1 << endl;
        return 0;
    }
    
    map<int, int> index_of_num;

    for (int i = 0; i < n; i++)
        index_of_num[A[i]] = i;

    vector<vector<int>> adj(n);

    for (int pos = n-1; pos >= 1; pos--) {

        int value = A[pos];

        for (int div = 1; div <= value/2; div++) {
            if (value % div == 0) {
                int index_of_div = index_of_num[div];
                adj[index_of_div].push_back(pos);
            }
        }

        adj[pos-1].push_back(pos);

    }

    cout << bfs(0, n-1, n, adj) << endl;
    return 0;

}

/*
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8 10 3 6 5 1 4 7 2 9
*/

Test 1

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Test 2

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Test 3

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Test 4

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Test 5

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Test 10

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Test 11

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Test 12

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Test 15

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Test 21

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Test 22

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Test 24

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Test 25

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Test 30

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Test 40

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Test 44

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Test 45

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Test 48

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Test 49

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Test 50

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Test 59

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Test 60

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Test 62

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Test 71

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