poj3461 hash字符串匹配

可以用KMP写，然而发现hash不用写很长，而且好理解。

定义一个匹配串的hash值为h[m] = c[1]b^ (m - 1) + c[2]b^(m - 2) + …… + c[m - 1]b + c[m]，然后向后推的时候h[i] = h[i - 1] - c[i - m] * pow(b,m) + c[i];

然后直接比较在模式串上子串的hash值是不是等于匹配串，是的话就匹配了。

b的选择一般是质数，为了溢出也是正数一般数都定义为usigned long long;

//#include<bits/stdc++.h>#include<iostream>#include<cstdio>#include<cstring>using namespace std;typedef long long ll;typedef unsigned long long ull;typedef pair<int,int> P;#define fi first#define se second#define INF 0x3f3f3f3f#define clr(x,y) memset(x,y,sizeof x)#define PI acos(-1.0)#define ITER set<int>::iteratorconst int Mod = 1e9 + 7;const int maxn = 1000000 + 10;const int N = 2;const ull M = 133;char s[maxn],t[maxn];ull p[maxn];int main(){    int Tcase;scanf("%d",&Tcase);    while(Tcase --)    {        scanf("%s%s",s,t);int len1 = strlen(s),len2 = strlen(t);        ull ah = 0, bh = 0;ull temp = 1;for(int i = 0; i < len1; i ++)temp *= M;        for(int i = 0;i < len1; i ++)ah = ah * M + s[i],bh = bh * M + t[i];        int ans = 0;        for(int i = len1 - 1; i < len2; i ++)        {            if(bh == ah)ans ++;bh = bh * M - temp * t[i - len1 + 1] + t[i + 1];        }        printf("%d\n",ans);    }    return 0;}