Oulipo 子串查找

思路

使用哈希值表示较长串的子串的值，直接比较哈希值是否相等

代码

#include <cstdio>
#include <cstring>
using namespace std;
char a[1000005], b[1000005];
unsigned int sum[1000005], sumb, bt[1000005];
int bs = 37, ans;
int lena, lenb;
unsigned int get(int st) {
    return sum[st + lenb - 1] - sum[st - 1] * bt[lenb];
}
int main() {
    scanf("%s %s", a + 1, b + 1);
    lena = strlen(a + 1), lenb = strlen(b + 1);
    bt[0] = 1;
    for (int i = 1; i <= lena; ++i) {
        sum[i] = sum[i - 1] * bs + a[i];
        bt[i] = bt[i - 1] * bs;
    }
    for (int i = 1; i <= lenb; ++i) {
        sumb = sumb * bs + b[i];
    }
    for (int i = 1; i + lenb - 1 <= lena; ++i) {
        if (get(i) == sumb) ans++;
    }
    printf("%d", ans);
    return 0;
}