URAL 1297（最长回文串）

题目：http://acm.timus.ru/problem.aspx?space=1&num=1297

题目大意：给你一个字符串，问你输出其中最长的回文串，如果有多个，就输出最早出现的那个。

思路：把那个字符串倒过来然后黏在后面，中间用一个不同的ASC码，然后做后缀数组，然后枚举中间位置i，对于每个i，要考虑回文串是奇数和偶数的情况，然后求 lcp，注意空间开两倍，RE了一次，挺基础的一个题目，然后就木有然后了。。= =

代码如下：

#include<cstdio>#include<cstring>#include<algorithm>using namespace std;const int MAXN = 1111<<1;char str[MAXN];int sa[MAXN],rank[MAXN],height[MAXN];int t[MAXN],t2[MAXN],c[MAXN];void cal_height(int n){    for(int i = 1;i <= n;i++) rank[sa[i]] = i;    int k = 0;    for(int i = 0;i < n;i++)    {        if(k) k--;        int j = sa[rank[i]-1];        while(str[i+k] == str[j+k]) k++;        height[rank[i]] = k;    }}void da(int n,int m){    int *x = t,*y = t2;    for(int i = 0;i < m;i++) c[i] = 0;    for(int i = 0;i < n;i++) c[x[i] = str[i]]++;    for(int i = 1;i < m;i++) c[i] += c[i-1];    for(int i = n-1;i >= 0;i--) sa[--c[x[i]]] = i;    for(int k = 1;k <= n;k <<= 1)    {        int p = 0;        for(int i = n-k;i < n;i++) y[p++] = i;        for(int i = 0;i < n;i++) if(sa[i] >= k) y[p++] = sa[i] - k;        for(int i = 0;i < m;i++) c[i] = 0;        for(int i = 0;i < n;i++) c[x[y[i]]]++;        for(int i = 1;i < m;i++) c[i] += c[i-1];        for(int i = n-1;i >=0;i--) sa[--c[x[y[i]]]] = y[i];        swap(x,y);        p = 1;        x[sa[0]] = 0;        for(int i = 1;i < n;i++)            x[sa[i]] = y[sa[i]] == y[sa[i-1]] && y[sa[i]+k] == y[sa[i-1]+k] ? p-1 : p++;        if(p >= n) break;        m = p;    }    cal_height(n-1);}int d[MAXN][20];void rmq_init(int n){    for(int i = 1;i <= n;i++)        d[i][0] = height[i];    for(int j = 1;(1<<j) <= n;j++)        for(int i = 1;i+(1<<j) <= n;i++)            d[i][j] = min(d[i][j-1],d[i+(1<<(j-1))][j-1]);}int rmq(int a,int b){    int k = 0;    while((1<<(k+1)) < (b-a+1)) k++;    return min(d[a][k],d[b-(1<<k)+1][k]);}int lcp(int a,int b){    a = rank[a],b =rank[b];    if(a > b) swap(a,b);    return rmq(a+1,b);}int main(){    while(~scanf("%s",str))    {        int n = strlen(str);        str[n] = 127;        for(int i = 0;i < n;i++)            str[n+1+i] = str[n-i-1];        int nn = (n<<1)+1;        str[nn] = 0;        //for(int i = 0;i<nn;i++)            //printf("%d\n",str[i]);        da(nn+1,256);        int kind = -1;        int ans_len = 0,pos;        rmq_init(nn);        for(int i = 0;i < n;i++)        {            int tmp;            if(i > 0)            {                tmp = lcp(i,2*n-(i-1))*2;                if(tmp > ans_len)                {                    ans_len = tmp;                    pos = i;                    kind = 0;                }                //printf("i = %d,tmp1 = %d\n",i,tmp);            }            tmp = 2*lcp(i+1,2*n-(i-1))+1;            if(tmp > ans_len)            {                ans_len = tmp;                pos = i;                kind = 1;            }            //printf("i = %d,tmp2 = %d\n",i,tmp);        }        //printf("ans_len = %d,pos = %d\n",ans_len,pos);        if(kind == 0)        {            int tt = ans_len/2;            for(int i = pos-tt;i <= pos+tt-1;i++)                printf("%c",str[i]);        }        else if(kind == 1)        {            int tt = (ans_len-1)/2;            for(int i = pos-tt;i <= pos+tt;i++)                printf("%c",str[i]);        }        puts("");    }    return 0;}