2019 cattle off more school eighth field B Beauty Values (contribution)

Links: https://ac.nowcoder.com/acm/contest/888/B

The meaning of problems: given a sequence, find all its subintervals Beauty (the number of kinds of section number). i <1e5, ai <1e5

Solution: Consider every kind of contribution to the answer, that is, considering the contribution of each number of the answer, look at this number appeared in the number of intervals.

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

const int maxn=1e5+5;
int n, a[maxn], pre[maxn];

int main()
{
    ios::sync_with_stdio(false); cin.tie(0);
    cin>>n;
    for(int i=1; i<=n; i++) cin>>a[i];
    ll ans=0;
    for(int i=1; i<=n; i++)
    {
        ans+=1ll*(i-pre[a[i]])*(n-i+1);
        pre[a[i]]=i;
    }
    cout<<ans;
    return 0;
}