后缀数组（最长回文子串）——ural 1297

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#include <stdio.h>  
#include <stdlib.h>  
#include <string.h>  
#include <math.h>  
#define MS(x, y) memset(x, y, sizeof(x))  
#define MIN(x, y) ((x)<(y)?(x):(y))
#define MAX(x, y) ((x)>(y)?(x):(y))
const int MAXN = 2000+10;  
const int INF = 1<<30;  
  
int dp[MAXN][20];
int wa[MAXN],wb[MAXN],wv[MAXN],ws[MAXN];  
int rank[MAXN],r[MAXN],sa[MAXN],height[MAXN];  
char str[MAXN];  
  
int cmp(int *r, int a, int b, int l)  
{  
    return r[a] == r[b] && r[a+l] == r[b+l];  
}  
  
void da(int *r, int *sa, int n, int m)  
{  
    int i, j, p, *x = wa, *y = wb, *t;  
  
    for(i=0; i<m; i++) ws[i] = 0;  
    for(i=0; i<n; i++) ws[x[i] = r[i]]++;  
    for(i=1; i<m; i++) ws[i] += ws[i-1];  
    for(i=n-1; i>=0; i--) sa[--ws[x[i]]] = i;  
  
    for(j=1,p=1; p<n; j<<=1, m=p){  
  
        for(p=0,i=n-j; i<n; i++) y[p++] = i;  
        for(i=0; i<n; i++) if(sa[i] >= j) y[p++] = sa[i] - j;  
  
        for(i=0; i<n; i++) wv[i] = x[y[i]];  
        for(i=0; i<m; i++) ws[i] = 0;  
        for(i=0; i<n; i++) ws[wv[i]]++;  
        for(i=1; i<m; i++) ws[i] += ws[i-1];  
        for(i=n-1; i>=0; i--) sa[--ws[wv[i]]] = y[i];  
  
        for(t=x,x=y,y=t,p=1,x[sa[0]]=0,i=1; i<n; i++)  
            x[sa[i]] = cmp(y, sa[i-1], sa[i], j) ? p-1 : p++;  
  
    }  
    return;  
}  
  
void calheight(int *r, int *sa, int n)  
{  
    int i, j, k = 0;  
    for(i=1; i<n; i++) rank[sa[i]] = i;  
    for(i=0; i<n-1; height[rank[i++]] = k)  
        for(k ? k-- : 0,j=sa[rank[i]-1]; r[i+k] == r[j+k]; k++);  
    return;  
}  

void caldp(int n)  
{  
	int i,j;
    for(i=1; i<n; i++)
        dp[i][0]=height[i];  

    for(j=1; j<=15; j++){ //注意这个要在外层  
        for(i=1; i<n; i++){  
            if(i+(1<<j)-1 < n){  
                dp[i][j]=MIN(dp[i][j-1], dp[i+(1<<(j-1))][j-1]);  
            }  
        }  
    }  
}  
  
int rmq(int l, int r)  
{  
    int k = int(log(double(r-l+1))/log(2.0));  
    return MIN(dp[l][k], dp[r-(1<<k)+1][k]);  
}  

int main()
{
	//freopen("in.txt", "r", stdin);
	while(~scanf("%s", str))
	{
		MS(dp, 0);
		MS(r, 0);
		MS(sa, 0);
		MS(rank, 0);
		MS(height, 0);
		MS(ws, 0);
		MS(wv, 0);
		MS(wa, 0);
		MS(wb, 0);
		
		int path = 0;
		int i, n = 0;
		int len = strlen(str);
		for(i=0; i<len; i++)
			r[n++] = (int)str[i] - (int)'A' + 1;
		path = n;
		r[n++] = 59;
		for(i=len-1; i>=0; i--)
			r[n++] = (int)str[i] - (int)'A' + 1;
		r[n++] = 0;

		da(r, sa, n, 60);
		calheight(r, sa, n);
		caldp(n);

		int Set = -1, Len = 0;
		int s = 0, l = 1, s1 = 0, l1 = 1, s2 = 0, l2 = 1;
		int left, right, ok;
		for(i=0; i<len; i++){
			ok = 0;
			if(i != len-1 && r[i] == r[i+1]){//回文串为偶数
				ok = 1;
				left = MIN(rank[i + 1], rank[n - 2 - i]);
				right = MAX(rank[i + 1], rank[n - 2 - i]);
				l1 = rmq(left + 1, right);
				s1 = i - l1 + 1;
				l1 = 2 * l1;
			}

			left = MIN(rank[i], rank[n - 2 - i]);
			right = MAX(rank[i], rank[n - 2 - i]);
			l2 = rmq(left + 1, right);
			s2 = i - l2 + 1;
			l2 = 2 * l2 - 1;

			l = l2; s = s2;
			if(ok) if(l1 > l2){
				l = l1; s = s1;
			}

			if(l == Len && s < Set) Set = s;
			else if(l > Len){
				Len = l;
				Set = s;
			}
		}
		for(i=Set; i < Len+Set; i++)
			printf("%c", r[i] + 'A' - 1);
		printf("\n");
	}
	return 0;
}