wenbao与字符串Hash

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http://begin.lydsy.com/JudgeOnline/problem.php?id=1729

N*M字符矩阵中找出至少两个相同的正方形，要求边长尽量长 

#include <iostream>
#include <algorithm>
using namespace std;

#define ll long long
const int maxn = 505;
const ll D = 97, D2 = 131;
int n, m;
ll p[maxn], p2[maxn], hs[maxn*maxn], h[maxn][maxn];
char a[maxn][maxn];

bool ok(int x){
    for(int i = 1; i <= n; ++i){
        ll tmp = 0;
        for(int j = 1; j < x; ++j) tmp = tmp*D + a[i][j], h[i][j] = 0;
        for(int j = x; j <= m; ++j) h[i][j] = tmp = tmp*D - p[x]*a[i][j-x] + a[i][j];
    }
    int num = 0;
    for(int i = x; i <= m; ++i){
        ll tmp = 0;
        for(int j = 1; j < x; ++j) tmp = tmp*D2 + h[j][i];
        for(int j = x; j <= n; ++j) hs[num++] = tmp = tmp*D2 - p2[x]*h[j-x][i] + h[j][i];
    }
    sort(hs, hs+num);
    for(int i = 1; i < num; ++i) if(hs[i] == hs[i-1]) return true;
    return false;
}
int main(){
    scanf("%d%d", &n, &m);
    for(int i = 1; i <= n; ++i){
        scanf("%s", a[i]+1);
        for(int j = 1; j <= m; ++j) a[i][j] -= 'a';
    }
    int l = 0, r = (n < m ? n : m), sum = 0;
    p[0] = 1, p2[0] = 1;
    for(int i = 1; i <= r; ++i) p[i] = p[i-1]*D, p2[i] = p2[i-1]*D2;
    while(l <= r){
        int mid = (l+r)>>1;
        if(ok(mid)) sum = mid, l = mid+1;
        else r = mid-1;
    }
    printf("%d
", sum);
    return 0;
}

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http://acm.scu.edu.cn/soj/problem.action?id=4438

#include <iostream>
#include <string.h>
using namespace std;

#define ll long long
const int maxn = 5e6+10;
char str[maxn], ss[maxn], st[maxn];
ll p, D = 97, a[maxn];

int main(){
    while(~scanf("%s%s", str, ss)){
        ll aim = 0;
        p = 1;
        int len1 = strlen(str), len2 = strlen(ss);
        for(int i = 0; i < len1; ++i){
            aim = aim*D + str[i]-'a';
            p *= D;
        }
        ll tmp = 0;
        int top = 0;
        for(int i = 0; i < len2; ++i){
            st[top] = ss[i];
            tmp = tmp*D + ss[i] - 'a';
            a[++top] = tmp;
            if(top >= len1 && tmp - a[top-len1]*p == aim){
                //cout<<i<<"***"<<endl;
                tmp = a[top-len1];
                top -= len1;
            }
        }
        st[top] = '';
        printf("%s
", st);
    }
    return 0;
}

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只有不断学习才能进步！