HDU 5763 (哈希 DP)

题目链接：点击这里

题意：某个子串可以用*代替，求一个字符串的所有表达方式。

用哈希（或者KMP）预处理每一个下标是不是子串的结尾，用dpi表示原串到i下标有多少种表达方式，转移就是

dpi={dpi−1dpi−1+dpi−len(子串)i不是子串的结尾i是子串的结尾

#include <cstring>#include <cstdio>#include <cmath>#include <algorithm>#include <iostream>using namespace std;#define maxn 100005#define mod 1000000007typedef unsigned long long ull;#define seed 131char P[maxn], T[maxn];long long dp[maxn];bool is[maxn];int n, m;#define hash Hashull hash[maxn];ull num;ull f (char a) {    return a-'a'+1;}#define pow Powull pow[maxn];void init () {    pow[0] = 1;    for (int i = 1; i < maxn; i++) pow[i] = pow[i-1]*seed;    num = 0;    for (int i = 0; i < m; i++) {        if (!i) num = f (P[i]);        else num = num*seed + f (P[i]);    }    for (int i = 0; i < m; i++) {        if (i == 0) hash[i] = f (T[i]);        else hash[i] = hash[i-1]*seed + f(T[i]);    }    for (int i = m; i < n; i++) {        hash[i] = (hash[i-1]-f (T[i-m])*pow[m-1])*seed + f (T[i]);    }}int DP (int i) {    if (i == -1)        return 1;    else         return dp[i];}int main () {    int t;    scanf ("%d", &t);    for (int kase = 1; kase <= t; kase++) {        scanf ("%s%s", T, P);        memset (dp, 0, sizeof dp);        n = strlen (T), m = strlen (P);        init ();        for (int i = 0; i < n; i++) {            if (hash[i] != num) {                dp[i] = DP (i-1);            }            else {                dp[i] = DP (i-m) + DP (i-1);                dp[i] %= mod;            }        }        printf ("Case #%d: %lld\n", kase, dp[n-1]);    }    return 0;}