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;}