Longest Palindromic Substring leetcode java
题目:
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.
题解:
第一种方法就是挨个检查,维护全局最长,时间复杂度为O(n3),会TLE。
代码如下:
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++) { int len = j - i; String curr = s.substring(i, j + 1); if (isPalindrome(curr)) { if (len > maxPalinLength) { longestPalindrome = curr; maxPalinLength = len; } } } } return longestPalindrome; } 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; }
第二种方法“是对于每个子串的中心(可以是一个字符,或者是两个字符的间隙,比如串abc,中心可以是a,b,c,或者是ab的间隙,bc的间隙,例如aba是回文,abba也是回文,这两种情况要分情况考虑)往两边同时进
行扫描,直到不是回文串为止。假设字符串的长度为n,那么中心的个数为2*n-1(字符作为中心有n个,间隙有n-1个)。对于每个中心往两边扫描的复杂
度为O(n),所以时间复杂度为O((2*n-1)*n)=O(n^2),空间复杂度为O(1)。”引自Code ganker(http://codeganker.blogspot.com/2014/02/longest-palindromic-substring-leetcode.html)
代码如下:
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
if (tmp.length() > longest.length())
longest = tmp;
// get longest palindrome with center of i, i+1
tmp = helper(s, i, i + 1);
if (tmp.length() > longest.length())
longest = tmp;
}
return longest;
}
// Given a center, either one letter or two letter,
// Find longest palindrome
public String helper(String s, int begin, int end) {
while (begin >= 0 && end <= s.length() - 1 && s.charAt(begin) == s.charAt(end)) {
begin--;
end++;
}
return s.substring(begin + 1, end);
}
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
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