Longest Palindromic Substring

提述：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.

写了一个算法，但是复杂度有点高。有时间补上低复杂度的算法。

这是一道求最长回文子串的题。刚开始感觉无从下手，一篇博客下面的评论给了一种思路：遍历字符串，对每一位进行判断是否有回文串出现。出现回文串的情况我分了三种来讨论：1.当前位置跟左边一位是一致的 2.当前位置跟右边一位是一致的 3. 当前位置的左边跟右边是一致的。用一个boolean值记录如果是情况3的话那么这个子串中间会单出一个字符。 剩下的就是王子串的两边递归直至左右两边的字符不相同为止。

代码：

package leetcode;public class LongestPalin { public String longestPalindrome(String s) { char [] ch = s.toCharArray(); int lenArray[] = new int [s.length()]; String [] stringA = new String[s.length()]; //初始化 for(int i=0;i<lenArray.length;i++){ lenArray[i]=0; } boolean isOdd=false; for(int i=1;i<s.length()-1;i++){   //先判断有没有可能 int p1=0; int p2=0; if(ch[i-1]==ch[i]){//左中相同 stringA[i]= String.valueOf(ch[i-1]); p1= i-2; p2 =i+1; while(p1>=0 && p2< ch.length){     if(ch[p1]==ch[p2]){    // lenArray[i]++;     stringA[i]=  stringA[i]+String.valueOf(ch[p1]);     p1--;     p2++;     }else break;  } } if(ch[i]==ch[i+1]){ //中右相同 stringA[i]= String.valueOf(ch[i]); p1= i-1; p2 =i+2; while(p1>=0 && p2<ch.length){     if(ch[p1]==ch[p2]){     stringA[i]=  stringA[i]+String.valueOf(ch[p1]);     p1--;     p2++;              }else {         break;     }  } } if(ch[i-1]==ch[i+1]){ //左右开始往下找 isOdd = true; p1= i-1; p2 =i+1; stringA[i]= String.valueOf(ch[i]); while(p1>=0 && p2< ch.length){      if(ch[p1]==ch[p2]){     stringA[i]=  stringA[i]+String.valueOf(ch[p1]);     p1--;     p2++;     }else break;  } } }  String tar = "";  for (String string : stringA) {if (string!= null && tar.length()< string.length()){tar= string;}} //构造palin  String res=""; if(isOdd){ for(int i=tar.length()-1;i>0;i--){ res= res+String.valueOf(tar.charAt(i)); } res += tar; return res; } else{    for(int i=tar.length()-1;i>=0;i--){ res= res+String.valueOf(tar.charAt(i)); } res += tar; return res; }           }    public static void main(String args[]){   LongestPalin lp  = new LongestPalin();  <pre name="code" class="java">          String str = "abccbasdfsczxvcasdfghjkkjhgfdsazxc";

//String str = "abccbasdfsczxvcasdfghjkkjhgfdsazxc"; //String str ="aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"; System.out.println( "answer:"+lp.longestPalindrome(str)); }}