牛牛的Link Power I

二阶差分

#include <bits/stdc++.h>
/*
#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/hash_policy.hpp>
*/
using namespace std;

const double eps = 1e-10;
const double pi = 3.1415926535897932384626433832795;
const double eln = 2.718281828459045235360287471352;

#define f(i, a, b) for (int i = a; i <= b; i++)
#define scan(x) scanf("%d", &x)
#define mp make_pair
#define pb push_back
#define lowbit(x) (x&(-x))

#define fi first
#define se second
#define SZ(x) int((x).size())
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define summ(a) (accumulate(all(a), 0ll))

typedef unsigned long long ull;
typedef pair<int,int> pii;
typedef vector<int> vi;

using ll=long long;
const int mod=1e9+7;
ll n,dif[(int)1e5+9],ans;
char s[(int)1e5+9]; 
int main()
{
    scanf("%lld%s",&n,s+1);
    for(int i=1;i<n;++i)if(s[i]=='1')dif[i+1]+=1;
    for(int i=1;i<=n;++i){
        dif[i]+=dif[i-1];
        if(dif[i]>mod)dif[i]-=mod;
    }
    for(int i=1;i<=n;++i){
        dif[i]+=dif[i-1];
        if(dif[i]>mod)dif[i]-=mod;
    }
    for(int i=1;i<=n;++i){
        if(s[i]=='1'){
            ans+=dif[i];
            if(ans>mod)ans-=mod;
        }
    }
    printf("%lld\n", ans);
    return 0;
}