5. 最长回文子串

s_new和数组p都要开大，不然爆掉😴

#define f(i, a, b) for (int i = a; i <= b; i++)
#define min(a, b) (a < b ? a : b)

char * longestPalindrome(char * s){
    char s_new[5000];
    int j = 2;
    s_new[0] = '$';
    s_new[1] = '#';
    f(i, 0, strlen(s) - 1)
    {
        s_new[j++] = s[i];
        s_new[j++] = '#';
    }
    s_new[j++] = '^';
    s_new[j] = '\0';
    int p[20000]={0};
    int center=0, right = 0,maxlen=0,maxcenter=0;
    f(i, 2, strlen(s_new))
    {
        p[i] = i >= right ? 1 : min(right - i, p[2 * center - i]);
        while(s_new[i-p[i]]==s_new[i+p[i]]){
            p[i]++;
        }
        if(i+p[i]>right){
            right = i + p[i];
            center = i;
        }
        if(maxlen<p[i]){
            maxlen = p[i];
            maxcenter = i; 
        }
    }   
    static char res[2000];
    j = 0;   
    f(i, (maxcenter - maxlen) / 2, (maxcenter + maxlen) / 2 - 2)
        res[j++] = s[i];
    res[j] = '\0';
    return res;
}