Remove the forbidden strings

// Java program to remove forbidden strings 
import java.io.*;

class rtf {
    
    // number of forbidden strings
    public static int n;
    
    // original string
    public static String z;

    // forbidden strings
    public static String s[] = new String[100];

    // to store original string 
    // as character array
    public static char w[];
    
    // letter to replace and occur
    // max number of times
    public static char letter;   
    
    // pre[] keeps record of the characters
    // of w that need to be changed
    public static boolean pre[] = new boolean[100];

    // Function to check if the particular
    // substring is present in w at position
    public static void verify(int position, int index)
    {
        int l = z.length();
        int k = s[index].length();
    
        // If length of substring from this 
        // position is greater than length
        // of w then return
        if (position + k > l)
            return;
        
        boolean same = true;
        for (int i = position; i < position + k; i++) {
        
            // n and n1 are used to check for 
            // substring without considering
            // the case of the letters in w 
            // by comparing the difference
            // of ASCII values
            int n, n1;
            char ch = w[i];
            char ch1 = s[index].charAt(i - position);
            if (ch >= 'a' && ch <= 'z')
                n = ch - 'a';
            else
                n = ch - 'A';
            if (ch1 >= 'a' && ch1 <= 'z')
                n1 = ch1 - 'a';
            else
                n1 = ch1 - 'A';
            if (n != n1)
                same = false;
        }
        
        // If same == true then it means a substring
        // was found starting at position therefore
        // all characters from position to length 
        // of substring found need to be changed 
        // therefore they need to be marked
        if (same == true) {
            for (int i = position; i < position + k; 
                                               i++)
                pre[i] = true;
            return;
        }
    }
    
    // Function performing calculations.
    public static void solve()
    {
        w = z.toCharArray();
        letter = 'd';
        int l = z.length();
        int p = letter - 'a';
        for (int i = 0; i < 100; i++)
            pre[i] = false;
    
        // To verify if any substring is
        // starting from index i
        for (int i = 0; i < l; i++) {
            for (int j = 0; j < n; j++)
                verify(i, j);
        }
    
        // Modifying the string w 
        // according to the rules
        for (int i = 0; i < l; i++) {
            if (pre[i] == true) {
                if (w[i] == letter)
                    w[i] = (letter == 'a') ? 'b' : 'a';
    
                // This condition checks 
                // if w[i]=upper(letter)
                else if (w[i] == (char)((int)'A' + p))
                    w[i] = (letter == 'a') ? 'B' : 'A';
    
                // This condition checks if w[i] 
                // is any lowercase letter apart 
                // from letter. If true replace
                // it with letter
                else if (w[i] >= 'a' && w[i] <= 'z')
                    w[i] = letter;
    
                // This condition checks if w[i] 
                // is any uppercase letter apart
                // from letter. If true then 
                // replace it with upper(letter).
                else if (w[i] >= 'A' && w[i] <= 'Z')
                    w[i] = (char)((int)'A' + p);
            }
        }
        System.out.println(w);
    }
    
    // Driver function for the program
    public static void main(String args[])
    {
        n = 3;
        s[0] = "etr";
        s[1] = "ed";
        s[2] = "ied";
        z = "PEtrUnited";
        solve();
    }
}