1040 Longest Symmetric String (25 分)

1040 Longest Symmetric String (25 分)

 

Given a string, you are supposed to output the length of the longest symmetric sub-string. For example, given Is PAT&TAP symmetric?, the longest symmetric sub-string is s PAT&TAP s, hence you must output 11.

Input Specification:

Each input file contains one test case which gives a non-empty string of length no more than 1000.

Output Specification:

For each test case, simply print the maximum length in a line.

Sample Input:

Is PAT&TAP symmetric?

Sample Output:

11

最大回文串，虽然可以暴力写，但是那样太无聊了。
马拉车来一波。

#include <bits/stdc++.h>
#define N 1000000
using namespace std;
int p[N];//保存当前点的半径


int Manacher(string s1){
    string s = "$#";
    for(int i = 0; i < s1.length(); i++){
        s += s1[i];
        s += '#';
    }
    int id=0;//表示到目前为止能到达最远的距离的点的位置
    int clen=0;//表示最长半径
    int mx = 0;//表示最远的距离
    for(int i = 1; i < s.length(); i++){
        p[i] = mx < i?1:min(mx-i, p[id*2-i]);
        while(s[i-p[i]] == s[i+p[i]])
            p[i]++;

        if(mx < i+p[i]){
            mx = i + p[i];
            id = i;
        }

        if(clen < p[i]){
            clen = p[i];
        }
    }
    return clen-1;
}


string s;
int main() {
    getline(cin,s);
    cout<<Manacher(s)<<endl;
    return 0;
}