马拉车

O(n)求字符串中的最长回文串的长度

char s[SIZE];
int len[SIZE*2];
char str[SIZE*2];
int manacher(){//预处理字符串，将字符串隔开,且开头和结尾字符串要不同,防止越界，如aaa预处理为@#a#a#a$
       int l = strlen(s);
    int ls = 0;
    str[ls++] = '@';
    for(int i=0;i<l;i++){
        str[ls++] = '#';
        str[ls++] = s[i];
    }
    str[ls++] = '#';
    str[ls++] = '$';
    str[ls] = '\0';
    int mx=0,po=0;
    int ans = -1;
    for(int i=1;i<ls-1;i++){
        if(i<mx) len[i] = min(mx-i,len[po*2-i]);
        else len[i] = 1;
        while(str[len[i]+i] == str[i-len[i]])
            len[i]++;
        if(len[i]+i>mx){
            mx = len[i]+i;
            po = i;
        }
        ans =max(ans,len[i]);
    }
    return ans - 1;
}