【C++】在一个字符串中寻找最长的回文子串

#include 
#include 
using namespace std;

int main(int argc, const char * argv[])
{
    //输入字符串
    string s;
    cin>>s;

	//开始遍历字符串
    vector<string> arr(0);
    for(int i = 0; i <s.length(); ++i)
    {
        //出现偶数长度的回文
        if((i + 1 < s.length()) && (s[i] == s[i+1]))
        {
            int l = 1;
            while ((i - l >= 0) && (i + 1 + l <s.length()) && (s[i-l] == s[i+1+l]))
            {
                ++l;
            }
            arr.push_back(s.substr(i-l+1, 2*l));
        }
        //出现奇数长度的回文
        if((i + 2 < s.length()) && (s[i] == s[i+2]))
        {
            int l = 1;
            while ((i - l >= 0) && (i + l + 2 <s.length()) && (s[i-l] == s[i+l+2]))
            {
                ++l;
            }
            arr.push_back(s.substr(i-l+1, 2*l+1));
        }
    }
    
    
    if(arr.size() > 0)
    {
    	//寻找最长的子串
        int max = 0;
        for(int i = 0; i < arr.size(); ++i)
        {
            if(arr[i].size() > max)
            {
                max = (int)arr[i].size();
            }
        }
        //输出最长的子串
        for(int i = 0; i < arr.size(); ++i)
        {
            if(arr[i].size() == max)
            {
                cout<<arr[i]<<endl;
            }
        }
    }
    else
    {
    	//若没有回文串则输出null
        cout<<"null"<<endl;
    }
}