迴文串

正反看都一樣的字串稱作迴文串
有AA，ABA兩種形式：

• AA
  形如 xxxxAAxxxx

• ABA
  形如 XXXXABAxxxx

A1040 Longest Symmetric String

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

題意

給出一個字串，求出最長的子迴文串有多長

思路

1. 迴文串一定是對稱的
2. 迴文串的對稱中心是 AA 或 ABA
3. 從對稱中心向兩邊延伸回文串，記錄下最長的長度

完整程式碼

#include <iostream>
#include <string>
#include <algorithm>
#include <vector>
#include <map>  
#include <set>
#include <cstdio>

using namespace std;


int longestSS(const string& s)
{
    int
 
 left, right;
    int length = s.length();
    int _max = 1;

    //#1 xxxxxaaxxxx
    for (size_t core = 0; core < length; core++)
    {
        left = core;
        right = core + 1;
        while (left >= 0 && right < length && s[left] == s[right])
        {
            int currentLength = right - left + 1;
            _max = currentLength > _max ? currentLength : _max;
            --left;
            ++right;
        }
    }

    //#2 xxxxxabaxxx
    for (size_t core = 0; core < length; core++)
    {
        left = core - 1;
        right = core + 1;
        while (left >= 0 && right < length && s[left] == s[right])
        {
            int currentLength = right - left + 1;
            _max = currentLength > _max ? currentLength : _max;
            --left;
            ++right;
        }
    }

    return _max;
}

int main()
{
    string s;
    getline(cin, s);
    cout << longestSS(s);
    system("pause");
    return 0;
}