# 最长回文子串
给你一个字符串 s，找到 s 中最长的回文子串。
示例 1：
输入：s = "babad"
输出："bab"
解释："aba" 同样是符合题意的答案。
示例 2：
输入：s = "cbbd"
输出："bb"
示例 3：
输入：s = "a"
输出："a"
示例 4：
输入：s = "ac"
输出："a"
提示：
1 <= s.length <= 1000s 仅由数字和英文字母（大写和/或小写）组成
以下错误的选项是？
## aop
### before
```cpp
#include
using namespace std;
```
### after
```cpp
int main()
{
Solution test;
string ret;
string s = "cbbd";
ret = test.longestPalindrome(s);
cout << ret << endl;
return 0;
}
```
## 答案
```cpp
class Solution
{
public:
string longestPalindrome(string s)
{
int len = s.size();
if (len == 0 || len == 1)
return s;
int start = 0;
int end = 0;
int mlen = 0;
for (int i = 0; i < len; i++)
{
int len1 = expendaroundcenter(s, i, i);
int len2 = expendaroundcenter(s, i, i + 1);
mlen = max(min(len1, len2), mlen);
if (mlen > end - start + 1)
{
start = i - (mlen - 1) / 2;
end = i + mlen / 2;
}
}
return s.substr(start, mlen);
}
private:
int expendaroundcenter(string s, int left, int right)
{
int L = left;
int R = right;
while (L >= 0 && R < s.length() && s[R] == s[L])
{
L--;
R++;
}
return R - L - 1;
}
};
```
## 选项
### A
```cpp
class Solution
{
public:
string longestPalindrome(string s)
{
int len = s.size();
if (len == 0 || len == 1)
return s;
int start = 0;
int max = 1;
vector> dp(len, vector(len));
for (int i = 0; i < len; i++)
{
dp[i][i] = 1;
if (i < len - 1 && s[i] == s[i + 1])
{
dp[i][i + 1] = 1;
max = 2;
start = i;
}
}
for (int l = 3; l <= len; l++)
{
for (int i = 0; i + l - 1 < len; i++)
{
int j = l + i - 1;
if (s[i] == s[j] && dp[i + 1][j - 1] == 1)
{
dp[i][j] = 1;
start = i;
max = l;
}
}
}
return s.substr(start, max);
}
};
```
### B
```cpp
class Solution
{
public:
string longestPalindrome(string s)
{
if (s.length() == 1)
return s;
string rev = s;
string res;
std::reverse(rev.begin(), rev.end());
if (rev == s)
return s;
int len = 0;
for (int i = 0; i < s.length(); i++)
{
string temp;
for (int j = i; j < s.length(); j++)
{
temp = temp + s[j];
if (len >= temp.length())
continue;
else if (rev.find(temp) != -1)
{
string q = temp;
std::reverse(q.begin(), q.end());
if (q == temp)
{
len = temp.length();
res = temp;
}
}
else
break;
}
temp = "";
}
return res;
}
};
```
### C
```cpp
class Solution
{
public:
string longestPalindrome(string s)
{
string res = "";
string temp = "";
for (int i = 0; i < s.length(); i++)
{
for (int j = i; j < s.length(); j++)
{
temp = temp + s[j];
string tem = temp;
std::reverse(tem.begin(), tem.end());
if (temp == tem)
res = res.length() > temp.length() ? res : temp;
}
temp = "";
}
return res;
}
};
```