poj 3974

来源：互联网发布：优先次序矩阵编辑：程序博客网时间：2024/06/08 03:06

Manacher算法, 用来解决最长回文子串一类问题，俗称马拉车算法，时间复杂度 O(n)

#include<cstdio>#include<cstdlib>#include<cstring>#include<iostream>#include<algorithm>const int maxl = 1000005;const char end[5] = "END";#define clear(arr) memset(arr,0,sizeof(arr))char ch[maxl], s[maxl<<1];int sl, par[maxl<<1];void init(){    int l = strlen(ch);    clear(s), clear(par);    for(int i = 0; i < l; i++)        s[i<<1|1] = ch[i], s[i<<1] = 255;    sl = l<<1|1, s[sl-1] = 255;}int solve(){    int ret = 0;    for(int i = 1, id = 0, mx = 0; i < sl-1; i++)    {        par[i] = (i < mx)?(std::min(mx-i,par[id*2-i])):1;#define Check(t) ((t)-par[t] >= 0 && (t)+par[t] < sl)               while(Check(i) && s[i-par[i]] == s[i+par[i]]) par[i]++;        if(i + par[i] > mx) id = i, mx = i + par[i];        ret = std::max(par[i] - 1, ret);    }    return ret;}int main(){    int T = 0;#ifndef ONLINE_JUDGE    freopen("poj3974.in","r",stdin);    freopen("poj3974.out","w",stdout);#endif    while(scanf("%s",ch) != EOF)    {        if(!strcmp(ch, end)) break;        printf("Case %d: ",++T);        init(), printf("%d\n",solve());    }#ifndef ONLINE_JUDGE    fclose(stdin);    fclose(stdout);#endif    return 0;}

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