hdu 3068 最长回文(manachar求最长回文子串）

题目连接：hdu 3068 最长回文

解题思路：通过manachar算法求最长回文子串，如果用遍历的话绝对超时。

 

#include <stdio.h>

#include <string.h>



const int N = 220005;



int rad[N];

char string[N], tmpstr[N];



int max(int a, int b) {

    return a > b ? a : b;

}

int min(int a, int b) {

    return a < b ? a : b;

}



int manachar() {

    int ans = 0, mix = 0, id = 0, len = strlen(string);

    for (int i = 1; i <= len; i++) {

	if (mix > i)

	    rad[i] = min(rad[2 * id - i], mix - i);

	else

	    rad[i] = 1;



	for ( ; string[i - rad[i]] == string[i + rad[i]]; rad[i]++) {

	    if (mix < i + rad[i]) {

		mix = i + rad[i];

		id = i;

	    }

	}



	ans = max(ans, rad[i]);

    }

    return ans - 1;

}



int main() {

    while (gets(tmpstr)) {

	int len = strlen(tmpstr), cnt = 0;

	string[cnt++] = '$';

	for (int i = 0; i <= len; i++) {

	    string[cnt++] = '#';

	    string[cnt++] = tmpstr[i];

	}

	getchar();

	printf("%d\n", manachar());

    }

    return 0;}