Longest Palindromic Substring leetcode java

題目：

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.

題解：

第一種方法就是挨個檢查，維護全域性最長，時間複雜度為O（n3），會TLE。

程式碼如下：

1 public String longestPalindrome(String s) {
2
3     int maxPalinLength = 0;
4     String longestPalindrome = null;
5     int length = s.length();
6
7     // check all possible sub strings
8     for (int i = 0; i < length; i++) {
9         for (int j = i + 1; j < length; j++) {
10             int len = j - i;
11             String curr = s.substring(i, j + 1);
12             if (isPalindrome(curr)) {
13                 if (len > maxPalinLength) {
14                     longestPalindrome = curr;
15                     maxPalinLength = len;
16                 }
17             }
18         }
19     }
20
21     return longestPalindrome;
22 }
23
24 public boolean isPalindrome(String s) {
25
26     for (int i = 0; i < s.length() - 1; i++) {
27         if (s.charAt(i) != s.charAt(s.length() - 1 - i)) {
28             return false;
29         }
30     }
31
32     return true;
33 }

第二種方法“是對於每個子串的中心（可以是一個字元，或者是兩個字元的間隙，比如串abc,中心可以是a,b,c,或者是ab的間隙，bc的間隙，例如aba是迴文，abba也是迴文，這兩種情況要分情況考慮）往兩邊同時進行掃描，直到不是迴文串為止。假設字串的長度為n,那麼中心的個數為2*n-1(字元作為中心有n個，間隙有n-1個）。對於每個中心往兩邊掃描的複雜度為O(n),所以時間複雜度為O((2*n-1)*n)=O(n^2),空間複雜度為O(1)。”引自Code ganker（http://codeganker.blogspot.com/2014/02/longest-palindromic-substring-leetcode.html）

程式碼如下：

1     public String longestPalindrome(String s) {
2         if (s.isEmpty()||s==null||s.length() == 1)
3             return s;
4
5         String longest = s.substring(0, 1);
6         for (int i = 0; i < s.length(); i++) {
7             // get longest palindrome with center of i
8             String tmp = helper(s, i, i);
9
10             if (tmp.length() > longest.length())
11                 longest = tmp;
12
13             // get longest palindrome with center of i, i+1
14             tmp = helper(s, i, i + 1);
15             if (tmp.length() > longest.length())
16                 longest = tmp;
17         }
18
19         return longest;
20     }
21
22     // Given a center, either one letter or two letter,
23     // Find longest palindrome
24     public String helper(String s, int begin, int end) {
25         while (begin >= 0 && end <= s.length() - 1 && s.charAt(begin) == s.charAt(end)) {
26             begin--;
27             end++;
28         }
29         return s.substring(begin + 1, end);
30     }

Reference:

http://www.programcreek.com/2013/12/leetcode-solution-of-longest-palindromic-substring-java/

http://codeganker.blogspot.com/2014/02/longest-palindromic-substring-leetcode.html

Longest Palindromic Substring leetcode java

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