Longest Palindromic Substring | Set 2

Given a string, find the longest substring which is palindrome. For example, if the given string is “forgeeksskeegfor”, the output should be “geeksskeeg”.

Recommended: Please solve it on “PRACTICE ” first, before moving on to the solution.

We have discussed dynamic programming solution in the previous post. The time complexity of the Dynamic Programming based solution is O(n^2) and it requires O(n^2) extra space. We can find the longest palindrome substring in (n^2) time with O(1) extra space. The idea is to generate all even length and odd length palindromes and keep track of the longest palindrome seen so far.

Step to generate odd length palindrome:
Fix a centre and expand in both directions for longer palindromes.

Step to generate even length palindrome
Fix two centre ( low and high ) and expand in both directions for longer palindromes.

C++

// A O(n^2) time and O(1) space program to  
// find the longest palindromic substring  
#include <bits/stdc++.h> 
using namespace std; 
  
// A utility function to print a substring str[low..high]  
void printSubStr(char* str, int low, int high)  
{  
    for( int i = low; i <= high; ++i )  
        cout << str[i];  
}  
  
// This function prints the longest palindrome substring (LPS)  
// of str[]. It also returns the length of the longest palindrome  
int longestPalSubstr(char *str)  
{  
    int maxLength = 1; // The result (length of LPS)  
  
    int start = 0;  
    int len = strlen(str);  
  
    int low, high;  
  
    // One by one consider every character as center point of  
    // even and length palindromes  
    for (int i = 1; i < len; ++i)  
    {  
        // Find the longest even length palindrome  
        // with center points as i-1 and i.  
        low = i - 1;  
        high = i;  
        while (low >= 0 && high < len && str[low] == str[high])  
        {  
            if (high - low + 1 > maxLength)  
            {  
                start = low;  
                maxLength = high - low + 1;  
            }  
            --low;  
            ++high;  
        }  
  
        // Find the longest odd length palindrome with center  
        // point as i  
        low = i - 1;  
        high = i + 1;  
        while (low >= 0 && high < len && str[low] == str[high])  
        {  
            if (high - low + 1 > maxLength)  
            {  
                start = low;  
                maxLength = high - low + 1;  
            }  
            --low;  
            ++high;  
        }  
    }  
  
    cout<<"Longest palindrome substring is: ";  
    printSubStr(str, start, start + maxLength - 1);  
  
    return maxLength;  
}  
  
// Driver program to test above functions  
int main()  
{  
    char str[] = "forgeeksskeegfor";  
    cout<<"\nLength is: "<<longestPalSubstr(str)<<endl;  
    return 0;  
}  
  
// This is code is contributed by rathbhupendra 

C

// A O(n^2) time and O(1) space program to find the longest palindromic substring 
#include <stdio.h> 
#include <string.h> 
  
// A utility function to print a substring str[low..high] 
void printSubStr(char* str, int low, int high) 
{ 
    for( int i = low; i <= high; ++i ) 
        printf("%c", str[i]); 
} 
  
// This function prints the longest palindrome substring (LPS) 
// of str[]. It also returns the length of the longest palindrome 
int longestPalSubstr(char *str) 
{ 
    int maxLength = 1;  // The result (length of LPS) 
  
    int start = 0; 
    int len = strlen(str); 
  
    int low, high; 
  
    // One by one consider every character as center point of  
    // even and length palindromes 
    for (int i = 1; i < len; ++i) 
    { 
        // Find the longest even length palindrome with center points 
        // as i-1 and i.   
        low = i - 1; 
        high = i; 
        while (low >= 0 && high < len && str[low] == str[high]) 
        { 
            if (high - low + 1 > maxLength) 
            { 
                start = low; 
                maxLength = high - low + 1; 
            } 
            --low; 
            ++high; 
        } 
  
        // Find the longest odd length palindrome with center  
        // point as i 
        low = i - 1; 
        high = i + 1; 
        while (low >= 0 && high < len && str[low] == str[high]) 
        { 
            if (high - low + 1 > maxLength) 
            { 
                start = low; 
                maxLength = high - low + 1; 
            } 
            --low; 
            ++high; 
        } 
    } 
  
    printf("Longest palindrome substring is: "); 
    printSubStr(str, start, start + maxLength - 1); 
  
    return maxLength; 
} 
  
// Driver program to test above functions 
int main() 
{ 
    char str[] = "forgeeksskeegfor"; 
    printf("\nLength is: %d", longestPalSubstr( str ) ); 
    return 0; 
} 

Java

// Java implementation of O(n^2) time and O(1) space method 
// to find the longest palindromic substring 
public class LongestPalinSubstring 
{ 
    // A utility function to print a substring str[low..high] 
    static void printSubStr(String str, int low, int high) { 
        System.out.println(str.substring(low, high + 1)); 
    } 
  
    // This function prints the longest palindrome substring  
    // (LPS) of str[]. It also returns the length of the   
    // longest palindrome  
    static int longestPalSubstr(String str) { 
        int maxLength = 1; // The result (length of LPS) 
  
        int start = 0; 
        int len = str.length(); 
  
        int low, high; 
  
        // One by one consider every character as center 
        // point of even and length palindromes 
        for (int i = 1; i < len; ++i)  
        { 
            // Find the longest even length palindrome with  
            // center points as i-1 and i. 
            low = i - 1; 
            high = i; 
            while (low >= 0 && high < len 
                    && str.charAt(low) == str.charAt(high)) { 
                if (high - low + 1 > maxLength) { 
                    start = low; 
                    maxLength = high - low + 1; 
                } 
                --low; 
                ++high; 
            } 
  
            // Find the longest odd length palindrome with  
            // center point as i 
            low = i - 1; 
            high = i + 1; 
            while (low >= 0 && high < len 
                    && str.charAt(low) == str.charAt(high)) { 
                if (high - low + 1 > maxLength) { 
                    start = low; 
                    maxLength = high - low + 1; 
                } 
                --low; 
                ++high; 
            } 
        } 
  
        System.out.print("Longest palindrome substring is: "); 
        printSubStr(str, start, start + maxLength - 1); 
  
        return maxLength; 
    } 
  
    // Driver program to test above function 
    public static void main(String[] args) { 
          
        String str = "forgeeksskeegfor"; 
        System.out.println("Length is: " +  
                           longestPalSubstr(str)); 
    } 
  
} 
// This code is contributed by Sumit Ghosh 

Python

# A O(n^2) time and O(1) space program to find the  
#longest palindromic substring 
  
# This function prints the longest palindrome substring (LPS) 
# of str[]. It also returns the length of the longest palindrome 
def longestPalSubstr(string): 
    maxLength = 1
  
    start = 0
    length = len(string) 
  
    low = 0
    high = 0
  
    # One by one consider every character as center point of  
    # even and length palindromes 
    for i in xrange(1, length): 
        # Find the longest even length palindrome with center 
    # points as i-1 and i. 
        low = i - 1
        high = i 
        while low >= 0 and high < length and string[low] == string[high]: 
            if high - low + 1 > maxLength: 
                start = low 
                maxLength = high - low + 1
            low -= 1
            high += 1
  
        # Find the longest odd length palindrome with center  
        # point as i 
        low = i - 1
        high = i + 1
        while low >= 0 and high < length and string[low] == string[high]: 
            if high - low + 1 > maxLength: 
                start = low 
                maxLength = high - low + 1
            low -= 1
            high += 1
  
    print "Longest palindrome substring is:", 
    print string[start:start + maxLength] 
  
    return maxLength 
  
# Driver program to test above functions 
string = "forgeeksskeegfor"
print "Length is: " + str(longestPalSubstr(string)) 
  
# This code is contributed by BHAVYA JAIN 

C#

// C# implementation of O(n^2) time  
// and O(1) space method to find the 
// longest palindromic substring  
using System; 
  
class GFG 
{ 
// A utility function to print  
// a substring str[low..high]  
public static void printSubStr(string str,   
                               int low, int high) 
{ 
    Console.WriteLine(str.Substring(low,  
                     (high + 1) - low)); 
} 
  
// This function prints the longest  
// palindrome substring (LPS) of str[].  
// It also returns the length of the  
// longest palindrome  
public static int longestPalSubstr(string str) 
{ 
    int maxLength = 1; // The result (length of LPS) 
  
    int start = 0; 
    int len = str.Length; 
  
    int low, high; 
  
    // One by one consider every  
    // character as center point 
    // of even and length palindromes  
    for (int i = 1; i < len; ++i) 
    { 
        // Find the longest even length  
        // palindrome with center points  
        // as i-1 and i.  
        low = i - 1; 
        high = i; 
        while (low >= 0 && high < len &&  
               str[low] == str[high]) 
        { 
            if (high - low + 1 > maxLength) 
            { 
                start = low; 
                maxLength = high - low + 1; 
            } 
            --low; 
            ++high; 
        } 
  
        // Find the longest odd length  
        // palindrome with center point as i  
        low = i - 1; 
        high = i + 1; 
        while (low >= 0 && high < len &&  
               str[low] == str[high]) 
        { 
            if (high - low + 1 > maxLength) 
            { 
                start = low; 
                maxLength = high - low + 1; 
            } 
            --low; 
            ++high; 
        } 
    } 
  
    Console.Write("Longest palindrome substring is: "); 
    printSubStr(str, start, start + maxLength - 1); 
  
    return maxLength; 
} 
  
// Driver Code 
public static void Main(string[] args) 
{ 
    string str = "forgeeksskeegfor"; 
    Console.WriteLine("Length is: " + 
                       longestPalSubstr(str)); 
} 
} 
  
// This code is contributed by Shrikant13 

PHP

<?php  
// A O(n^2) time and O(1) space program to find the longest palindromic substring 
   
// A utility function to print a substring str[low..high] 
function printSubStr($str, $low, $high) 
{ 
    for( $i = $low; $i <= $high; ++$i ) 
        echo $str[$i]; 
} 
   
// This function prints the longest palindrome substring (LPS) 
// of str[]. It also returns the length of the longest palindrome 
function longestPalSubstr($str) 
{ 
    $maxLength = 1;  // The result (length of LPS) 
   
    $start = 0; 
    $len = strlen($str); 
   
    // One by one consider every character as center point of  
    // even and length palindromes 
    for ($i = 1; $i < $len; ++$i) 
    { 
        // Find the longest even length palindrome with center points 
        // as i-1 and i.   
        $low = $i - 1; 
        $high = $i; 
        while ($low >= 0 && $high < $len && $str[$low] == $str[$high]) 
        { 
            if ($high - $low + 1 > $maxLength) 
            { 
                $start = $low; 
                $maxLength = $high - $low + 1; 
            } 
            --$low; 
            ++$high; 
        } 
   
        // Find the longest odd length palindrome with center  
        // point as i 
        $low = $i - 1; 
        $high = $i + 1; 
        while ($low >= 0 && $high < $len && $str[$low] == $str[$high]) 
        { 
            if ($high - $low + 1 > $maxLength) 
            { 
                $start = $low; 
                $maxLength = $high - $low + 1; 
            } 
            --$low; 
            ++$high; 
        } 
    } 
   
    echo "Longest palindrome substring is: "; 
    printSubStr($str, $start, $start + $maxLength - 1); 
   
    return $maxLength; 
} 
   
// Driver program to test above functions 
  
$str = "forgeeksskeegfor"; 
echo "\nLength is: ". longestPalSubstr( $str ) ; 
return 0; 
?> 

Output:

Longest palindrome substring is: geeksskeeg
Length is: 10

Time complexity: O ( n^2 ) where n is the length of input string.
Auxiliary Space: O ( 1 )

We will soon be adding more optimized method as separate post.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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Recommended: Please solve it on “PRACTICE ” first, before moving on to the solution.

C++

C

Java

Python

C#

PHP

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