LeetCode [5]. Longest Palindromic Substring, longestsubstring

LeetCode [5]. Longest Palindromic Substring, longestsubstring

Longest Palindromic Substring

I. Questions: 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.
The topic requires the maximum symmetric substring of a given string. For example, the maximum symmetric substring of "aaabccbacc" is "abccba ". Ii. Train of Thought: Compare each character or two characters in the center to the two sides and determine whether it is a symmetric subcharacter. The idea is simple. The two situations are as follows:
Figure 1 and two symmetric conditions. For symmetric strings, the above two conditions may occur. One is A "even symmetric", and the middle line is symmetric. The other is B "odd symmetric", with A character in the middle as the center symmetric. Always scan from 0 to the end of the string through I, move forward 0.5 each time, and take I as the center, scanning to both ends for judgment. The time complexity is O (n ^ 2), and the worst case is that all characters in the entire string are the same. Therefore, each scan has to reach the end. Iii. Java program

</Pre> <pre name = "code" class = "java"> public class Solution {public String longestPalindrome (String s) {int down = 0, up = 0, count = 0; // record the upper and lower intervals of the string and the length of the string int sl = s. length (); int maxl = 0; int subDown = 0, subUp = 0; // upper and lower intervals of the target substring String subS = new String (""); if (sl = 0) return ""; // 1. locate the middle position and upper and lower limits of the target substring for (double I = 0; I <= sl-1; I = I + 0.5) {down = (int) Math. floor (I); up = (int) Math. ceil (I); if (I % 1 = 0.5) & sl! = 0) // determine the status of I (A or B) {count = 0;} else {count = 1; down --; up ++;} // 2. scan the while (! (Down <0 | up> sl-1) {if (s. charAt (down )! = (S. charAt (up) {break; // 2.1. if the I-centric symmetric two-character characters are not equal, the scan jumps out} else {// 2.2. if the I-centered symmetric two-character characters are equal, continue to move count + = 2; down --; up ++ ;}}/3. update the longest string if (count> maxl) {maxl = count; subDown = down; subUp = up ;}// 3.1 because the string is moved before being judged during scanning, in fact, one unit is moved before and after the stop. Here, the unit is restored to subDown ++; subUp --; subS = s. substring (subDown, subUp + 1); return subS; // return symmetric substring }}

Extended learning: This article introduces several methods of Java Longest Palindromic Substring (Longest return string)

Leetcode-Longest Palindromic Substring (Java)