codeforces 165C Another Problem on Strings 双指针

题意：求对于给定01字符串，有多少个子串，其恰好包含了k个1

双指针，把每个1都找出来，统计这个1之后有多少个0

然后用乘法原理计算

#include <iostream>
#include <cstring>
#include <cstdio>
#include <algorithm>
#define rep(i, j, k) for(int i = j; i <= k; i++)

using namespace std;

typedef long long lo;

char buff[1001000];
lo l[1001000];

int main() {
lo sum=0,k, count=0, ptr=0;
cin >> k >> buff;
lo n=strlen(buff);
for (lo i=0;i<n;i++) {
l[ptr]++;
if (buff[i]=='1') ptr++;
}

l[ptr]++;
if (k==0)
for (lo i=0;i<=ptr;i++)
(count+=(l[i]-1)*(l[i]-2)/2 + l[i]-1);
else
for (lo i=0;i+k<=ptr;i++)
count+=(l[i])*(l[i+k]);
cout << count;
return 0;
}