Longest Palindromic Substring
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reference http://www.programcreek.com/2013/12/leetcode-solution-of-longest-palindromic-substring-java/
soln1 O(n^2) time, O(1) space
public class Solution {
public String longestPalindrome(String s) {
if(s==null) return null;
if(s.length()==1) return s;
String longest = s.substring(0,1); // because at least there are 2 elements in s
for(int i=0;i<s.length();i++){
String tmp = helper(s, i, i); // 中心对称外延,延伸出来的tmp为奇数位长度
if(longest.length()<tmp.length())
longest = tmp;
tmp = helper(s,i, i+1); //中心对称外延,tmp为偶数长度
if(longest.length()<tmp.length())
longest = tmp;
}
return longest;
}
public String helper(String s, int b, int e){
while(b>=0 && e< s.length() && s.charAt(b)==s.charAt(e)){
b--;
e++;
}
return s.substring(b+1, e);
}
}
soln2 DP
http://www.geeksforgeeks.org/longest-palindrome-substring-set-1/
http://www.programcreek.com/2013/12/leetcode-solution-of-longest-palindromic-substring-java/
会lte, 代码如下:
String res = null;
int len = s.length();
int[][] table = new int[len][len]; // a boolean table, record from i to j is there a palindromic string?
int MaxLen = 1;
for(int i=0;i<len;i++){
table[i][i]=1;
}
for(int i=0;i<len-1;i++){
if(s.charAt(i)==s.charAt(i+1)){
// MaxLen=2;
table[i][i+1]=1;
res = s.substring(i,i+2);
}
// else{
// table[i][i+1]=0;
// }
}
for(int L=3; L<=len; L++){
for(int i=0;i<len-L+1 ;i++){
int j = L+i-1;
if( s.charAt(i)==s.charAt(j) && table[i+1][j-1]==1){
table[i][j]=1;
if( L>MaxLen ){
MaxLen=L;
res = s.substring(i,j+1);
}
}else{
table[i][j]=0;
}
}
}
return res;
}
}
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