Leetcode 5. Longest Palindromic Substring

Question

Given a string S, find the longest palindromic substring in S. You may assume that the maximum length of S is 1000, and there exists one unique longest palindromic substring.

code

常规方式

/*
常规方法
*/
public String longestPalindromeC(String s) {
if (s == null || s.length() <= 1 || new StringBuilder(s).reverse().toString().equals(s)) {
return s;
}

String result = "";
for (int i = 1; i < s.length(); i++) {
int j = 1;
//奇数对称情况 比如aba
for (j = 1; i + j < s.length() && i - j >= 0 && s.charAt(i + j) == s.charAt(i - j); j++) ;
String temp1 = s.substring(i - j + 1, i + j);
if (temp1.length() > result.length()) {
result = temp1;
}
//偶数情况 aa类型
if (s.charAt(i) == s.charAt(i - 1)) {
for (j = 1; i + j < s.length() && i - j - 1 >= 0 && s.charAt(i + j) == s.charAt(i - j - 1); j++) ;
String temp2 = s.substring(i - j, i + j);
if (temp2.length() > result.length()) {
result = temp2;
}
}
}
return result;
}

DP方式

/*
标记数组加DP方法
*/
public String longestPalindromeDP(String s) {
if (s == null || s.length() == 0) {
return s;
}
boolean[][] flag = new boolean[s.length()][s.length()];
int start = 0;
int end = 0;

for (int i = 0; i < s.length(); i++) {
for (int j = 0; j <= i; j++) {
flag[j][i] = s.charAt(j) == s.charAt(i) && (i - j <= 2 || flag[j + 1][i - 1]);
if (flag[j][i]) {
if (end - start < i - j) {
start = j;
end = i;
}
}
}
}
return s.substring(start, end + 1);
}

优化DP

/*
DP方法
*/
public String longestPalindromeDPB(String s) {
int[] p = new int[2048];
StringBuilder t = new StringBuilder("$");
for (int i = 0; i < s.length(); ++i) {
t.append('#');
t.append(s.charAt(i));
}
t.append("#_");
// mx为已判断回文串最右边位置，id为中间位置，mmax记录p数组中最大值
int mx = 0, id = 0, mmax = 0;
int right = 0;
for (int i = 1; i < t.length() - 1; i++) {
p[i] = mx > i ? Math.min(p[2 * id - i], mx - i) : 1;
while (t.charAt(i + p[i]) == t.charAt(i - p[i]))
p[i]++;
if (i + p[i] > mx) {
mx = i + p[i];
id = i;
}
if (mmax < p[i]) {
mmax = p[i];
right = i;
}
}
// 最长为mmax - 1
return s.substring(right / 2 - mmax / 2, right / 2 - mmax / 2 + mmax - 1);
}

马拉车算法

马拉车算法