[leetcode]Longest Palindromic Substring
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问题描述:
Given a string S, find the longest palindromic substring inS. You may assume that the maximum length of S is 1000, and there exists one unique longest palindromic substring.
基本思想:
本题的navie解法是对每个(i,j) i<=j 测试是否是回文串,并记录最大的情况。O(n^3)
如果从回文串的中心考虑往两边找,则只需要O(n^2)的复杂度。此方法就是将每个点作为回文串中心去判定,记录并保持最长的回文字串。为了处理方便,可以在字符串中每两个字符见插入一个特殊字符"#",这样就可以将偶数回文串和奇数回文串的处理形式统一起来。
代码:
public String longestPalindrome(String s) { //Java if(s.length() <= 1) return s; String tmpStr = "#"; for(int i = 0; i <s.length(); i++) tmpStr += s.charAt(i) +"#"; String max = ""; for(int i = 1; i<tmpStr.length()-1; i=i+1){ int toleft = i-1; int toright = i+1; while(toleft >=0 && toright <tmpStr.length()){ if(tmpStr.charAt(toleft) != tmpStr.charAt(toright)) break; else { toleft--; toright++; } } if(toright - toleft -2 > max.length()) max = tmpStr.substring(toleft+1,toright); } return max.replaceAll("#",""); }
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