B - Trained?

B - Trained?

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Time limit : 2sec / Memory limit : 256MB

Score : 200 points

Problem Statement

Takahashi wants to gain muscle, and decides to work out at AtCoder Gym.

The exercise machine at the gym has N buttons, and exactly one of the buttons is lighten up. These buttons are numbered 1 through N. When Button i is lighten up and you press it, the light is turned off, and then Button ai will be lighten up. It is possible that i=ai. When Button i is not lighten up, nothing will happen by pressing it.

Initially, Button 1 is lighten up. Takahashi wants to quit pressing buttons when Button 2 is lighten up.

Determine whether this is possible. If the answer is positive, find the minimum number of times he needs to press buttons.

Constraints

• 2≤N≤105
• 1≤ai≤N

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Input

Input is given from Standard Input in the following format:

N
a1
a2
:
aN

Output

Print −1 if it is impossible to lighten up Button 2. Otherwise, print the minimum number of times we need to press buttons in order to lighten up Button 2.

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Sample Input 1

Copy

3
3
1
2

Sample Output 1

Copy

2

Press Button 1, then Button 3.

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Sample Input 2

Copy

4
3
4
1
2

Sample Output 2

Copy

-1

Pressing Button 1 lightens up Button 3, and vice versa, so Button 2 will never be lighten up.

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Sample Input 3

Copy

5
3
3
4
2
4

Sample Output 3

Copy

3

#include <iostream>
using namespace std;
int main()
{
    int n;
    int a[100000+100];
    int b[100000+100];
    while(cin>>n){
        memset(b,-1,sizeof(b));
        for(int i=1;i<=n;i++){
            cin>>a[i]; b[i]=0;
        }
        int flag=0; int k=0;
        int i=1;
        while(1){
            if(b[i]==0){
                b[i]=1; k++;
                i=a[i];
                if(i==2){
                  flag=1; break;
                }
            }
            else if(b[i]==1){
                flag=0; break;
            }
        }
        if(flag==0)
            cout<<-1<<endl;
        else
            cout<<k<<endl;
    }
    return 0;
}