求最长回文字符串

public class LongestPalindrome {
// 暴力算法
public String longestPalindrome(String s) {
if (s == null)
return null;
if (s.length() <= 1)
return s;
String sub = null;
int len = 0;
for (int i = 0; i < s.length(); i++)
　　　　　{
for (int j = i; j < s.length(); j++)
　　　　　　　{
String tmp = s.substring(i, j + 1);
if (isPalindrome(tmp) && tmp.length() > len)
　　　　　　　　　　{
sub = tmp;
len = tmp.length();
}
}
}
return sub;
}

public boolean isPalindrome2(String s) {
int len = s.length();
if (len % 2 == 1) {
// 这样写是很愚蠢的。因为既然知道了i是可以用总长度-i得到j，而不是让i和j同时滑动。
for (int i = 0, j = len - 1; i < len / 2 && j > len / 2; i++, j--) {
if (s.charAt(i) != s.charAt(j))
return false;
}
return true;
} else {
for (int i = 0, j = len - 1; i < len / 2 && j > len / 2; i++, j--) {
if (s.charAt(i) != s.charAt(j))
return false;
}
return true;
}
}

public boolean isPalindrome(String s) {
for (int i = 0; i < s.length() - 1; i++) {
if (s.charAt(i) != s.charAt(s.length() - 1 - i))
return false;
}
return true;
}

// 动态规划算法
public String longestPalindrome1(String s) {
if (s == null)
return null;
if (s.length() <= 1)
return s;
int maxLen = 0;
String longestStr = null;
int length = s.length();
int[][] table = new int[length][length];
// 单个字符是回文字符串
for (int i = 0; i < length; i++) {
table[i][i] = 1;
maxLen = 1;
}
// 两个相同字符也是回文字符串
for (int i = 0; i < length - 1; i++) {
if (s.charAt(i) == s.charAt(i + 1)) {
table[i][i + 1] = 1;
longestStr = s.substring(i, i + 2);
maxLen = 2;
}
}
// 3个字符或3个以上字符串判断,动态规划思想。
for (int L = 3; L <= length; L++)// L代表子字符串长度
{
for (int i = 0; i <= length - L; i++) {
int j = i + L - 1;
if (s.charAt(i) == s.charAt(j)) {
table[i][j] = table[i + 1][j - 1];
if (table[i][j] == 1 && L > maxLen) {
longestStr = s.substring(i, j + 1);
maxLen = L;
} else
table[i][j] = 0;
}
}
}
return longestStr;
}
//中心扩展法，要考虑奇对称和偶对称
public String longestPalindrome2(String s)
{
if (s.isEmpty())
{
return null;
}
if (s.length() == 1)
{
return s;
}
String longest = s.substring(0, 1);
for (int i = 0; i < s.length(); i++)
{
//奇扩展
String tmp = centerExpend(s, i, i);
if (tmp.length() > longest.length())
{
longest = tmp;
}

//偶扩展
tmp = centerExpend(s, i, i + 1);
if (tmp.length() > longest.length())
{
longest = tmp;
}
}
return longest;
}

public String centerExpend(String s, int begin, int end) {
while (begin >= 0 && end <= s.length() - 1
&& s.charAt(begin) == s.charAt(end))
{
begin--;
end++;
}
String subS = s.substring(begin + 1, end);
return subS;
}

public static void main(String[] args)
{
String s = "abb";
LongestPalindrome lp = new LongestPalindrome();
System.out.println(lp.longestPalindrome2(s));
}
}