HDU 3068 最长回文

Manacher算法练笔，O(n)求最长回文子串。

参考资料：http://blog.csdn.net/ggggiqnypgjg/article/details/6645824

http://www.felix021.com/blog/read.php?2040

后缀数组和拓展KMP也可以求，不过时间复杂度都是O(nlogn)。

#include <cstdio>
#include <cstring>

const int MAXN = 110010;

char s[MAXN];
char str[ MAXN << 1 ];
int p[ MAXN << 1 ];
int newL, n;

int min( int a, int b )
{
    return a < b ? a : b;
}

int max( int a, int b )
{
    return a > b ? a : b;
}

void init()
{
    int i;
    str[0] = '?';
    str[1] = '#';
    newL = 2;
    for ( i = 1; s[i]; ++i )
    {
        str[ newL++ ] = s[i];
        str[ newL++ ] = '#';
    }
    str[ newL ] = '\0';
    n = newL;

    return;
}

void DP()
{
    int maxID = 0, id;
    for ( int i = 1; i < newL; ++i )
    {
        if ( maxID > i ) p[i] = min( p[ (id << 1) - i ], p[id] + id - i );
        else p[i] = 1;

        while ( str[ i + p[i] ] == str[ i - p[i] ] ) ++p[i];
        if ( p[i] + i > maxID )
        {
            maxID = p[i] + i;
            id = i;
        }
    }
    return;
}

int main()
{
    while ( ~scanf( "%s", &s[1] ) )
    {
        init();
        DP();

        int maxL = 0;
        for ( int i = 1; i < n; ++i )
            maxL = max( maxL, p[i] - 1 );
        printf( "%d\n", maxL );
    }
    return 0;
}