Codeforces 21C stripe 2 Constructor

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#include <cstdio>#include <cstring>#include <algorithm>#include <vector>#include <iostream>#include <map>#include <set>#include <math.h>using namespace std;#define inf 10000000#define N 100050#define ll __int64ll n;ll a[N], lsum[N], rsum[N];ll lok[N], rok[N];int main(){ll i, j;while(cin>>n) {ll sum = 0;bool siz = false;for(i=1;i<=n;i++)cin>>a[i], sum += a[i], siz |= a[i];if(!siz) {cout<<(n-1)*(n-2)/2<<endl;continue;}if(sum%3){puts("0");continue;}sum/=3;lsum[0] = 0;for(i=1;i<=n;i++)lsum[i] = lsum[i-1]+a[i];rsum[n+1] = 0;for(i=n;i;i--)rsum[i] = rsum[i+1]+a[i];memset(lok, 0, sizeof lok);memset(rok, 0, sizeof rok);for(i=1;i<=n;i++) {if(lsum[i]==sum){lok[i] = 1;}//if(i==n)lok[i]=0;//lok[i] += lok[i-1];}for(i=n;i;i--) {if(rsum[i]==sum){rok[i] = 1;}rok[i]+=rok[i+1];}ll ans = 0;for(i = 1; i+2<=n; i++)ans += lok[i] * rok[i+2];//ll ans = (lok[n]*(lok[n]-1))/2;cout<<ans<<endl;}return 0;}/*7-1 1 -1 1 -1 1 09-5 -2 1 1 5 0 -4 4 01003 0 -5 2 -3 -1 -1 0 -2 -5 -4 2 1 2 -2 -1 -1 -4 3 -1 -3 -1 5 0 -4 -4 -1 0 -2 -2 0 1 -1 -2 -1 -5 -4 -2 3 1 -3 0 -1 1 0 -1 2 0 -2 -1 -3 1 -2 2 3 2 -3 -5 2 2 -2 -2 1 2 -2 -1 3 0 -4 7 -2 2 1 4 -9 -1 -2 -1 0 -1 0 -2 -2 -1 1 1 -4 2 -3 -3 7 1 1 -3 -7 0 -2 0 5 -2*/