longest_palindrome_subset.cpp

//
// A dynamic programming solution for longest palindr.
// This code is adopted from following link
// http://www.leetcode.com/2011/11/longest-palindromic-substring-part-i.html
//
//
// The All ▲lgorithms Project
//
// https://allalgorithms.com/strings
// https://github.com/allalgorithms/cpp
//
// Contributed by: Tushar Kanakagiri
// Github: @tusharkanakagiri
//
#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
// of str[].
// It also returns the length of the longest palindrome
int longestPalSubstr(char *str)
{
    int n = strlen(str); // get length of input string

    // table[i][j] will be false if substring str[i..j]
    // is not palindrome.
    // Else table[i][j] will be true
    bool table[n][n];
    memset(table, 0, sizeof(table));

    // All substrings of length 1 are palindromes
    int maxLength = 1;
    for (int i = 0; i < n; ++i)
        table[i][i] = true;

    // check for sub-string of length 2.
    int start = 0;
    for (int i = 0; i < n - 1; ++i)
    {
        if (str[i] == str[i + 1])
        {
            table[i][i + 1] = true;
            start = i;
            maxLength = 2;
        }
    }

    // Check for lengths greater than 2. k is length
    // of substring
    for (int k = 3; k <= n; ++k)
    {
        // Fix the starting index
        for (int i = 0; i < n - k + 1; ++i)
        {
            // Get the ending index of substring from
            // starting index i and length k
            int j = i + k - 1;

            // checking for sub-string from ith index to
            // jth index iff str[i+1] to str[j-1] is a
            // palindrome
            if (table[i + 1][j - 1] && str[i] == str[j])
            {
                table[i][j] = true;

                if (k > maxLength)
                {
                    start = i;
                    maxLength = k;
                }
            }
        }
    }

    printf("Longest palindrome substring is: ");
    printSubStr(str, start, start + maxLength - 1);

    return maxLength; // return length of LPS
}

// Driver program to test above functions
int main()
{
    char str[] = ""; //Enter string here
    printf("\nLength is: %d\n", longestPalSubstr(str));
    return 0;
}