Why this Code giving Runtime error on testcase 4 on codeforces

#1

#include<bits/stdc++.h>
using namespace std;

#define ll long long

int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);

ll n;
cin >> n;

vector<ll> a(n + 1);

for (ll i = 1; i <= n; i++)
	cin >> a[i];

vector<ll> lastOcc(n + 1, 0);
vector<ll> ans(n + 1, 0);

ans[0] = 0;
double sum = 0.0;

for (ll i = 1; i <= n; i++)
{
	ans[i] = ans[i - 1] + (i - lastOcc[a[i]]);
	lastOcc[a[i]] = i;

	sum = sum + ans[i];
}

double expected = 0.0;
expected = (2 * (sum - n) + n) / (n * n * 1.0);

cout << fixed << setprecision(6) << expected << endl;

return 0;

}

#2

Hello @devang_11912089,

Let me give you a test case where your code gives runtime error,

N = 1
array = [ 1000000 ]

Now in this line, lastOcc[a[i]] = lastOcc[1000000] => trying to access memory that never existed which gives runtime error (Size of lastOcc is 2).

ans[i] = ans[i - 1] + (i - lastOcc[a[i]]);
lastOcc[a[i]] = i;

#3

@devang_11912089,

Instead of creating arrays of size n+1, you should make it 1000001 (1e6+1)

1 Like
#4

@devang_11912089,

Is it working now?

#5

Yes it’s working now

#6

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