Core Concepts for String Operations

Java Input/Output Basics

• For reading from standard input: Scanner sc = new Scanner(System.in);
• For writing to standard output: System.out.println();

String Characteristics in Java

• Strings are immutable objects in Java.
• The length of a string can be obtained using the length() method: s.length().

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Problem 1: Reversing a Character Array

Implement a function that modifies a character array in-place to reverse its order.

Solution Implementation

public class ReverseCharacterSequence {
    public void invertArray(char[] data) {
        int startIdx = 0;
        int endIdx = data.length - 1;
        while(startIdx < endIdx) {
            char swap = data[startIdx];
            data[startIdx] = data[endIdx];
            data[endIdx] = swap;
            startIdx++;
            endIdx--;
        }
    }
}

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Problem 2: Custom String Reversal by Interval

Given a string s and an integer k, reverse the first k characters for every 2k characters. If fewer than k characters remain, reverse all remaining characters.

Solution Implementation

public class IntervalReversal {
    public String modifyString(String inputStr, int interval) {
        char[] charSequence = inputStr.toCharArray();
        for (int blockStart = 0; blockStart < charSequence.length; blockStart += 2 * interval) {
            int low = blockStart;
            int high = Math.min(blockStart + interval - 1, charSequence.length - 1);
            while (low < high) {
                char interim = charSequence[low];
                charSequence[low] = charSequence[high];
                charSequence[high] = interim;
                low++;
                high--;
            }
        }
        return new String(charSequence);
    }
}

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Problem 3: Digit Replacement in Strings

Read a string from standard input. Replace every digit (0-9) with the word "number". Return the modified string. This problem emphasizes ACM-style input/output hendling.

Solution Implementation

import java.util.Scanner;

public class DigitSubstitution {
    public static void main(String[] args) {
        Scanner inputReader = new Scanner(System.in);
        String original = inputReader.nextLine();
        char[] originalChars = original.toCharArray();
        
        int digitCount = 0;
        for (char ch : originalChars) {
            if (ch >= '0' && ch <= '9') {
                digitCount++;
            }
        }
        
        int newLength = originalChars.length + digitCount * 5;
        char[] resultArray = new char[newLength];
        
        for(int newIndex = newLength - 1, origIndex = originalChars.length - 1; newIndex >= 0; newIndex--, origIndex--) {
            char currentChar = originalChars[origIndex];
            if (currentChar > '9' || currentChar < '0') {
                resultArray[newIndex] = currentChar;
            } else {
                resultArray[newIndex] = 'r';
                resultArray[newIndex-1] = 'e';
                resultArray[newIndex-2] = 'b';
                resultArray[newIndex-3] = 'm';
                resultArray[newIndex-4] = 'u';
                resultArray[newIndex-5] = 'n';
                newIndex -= 5;
            }
        }
        System.out.println(new String(resultArray));
        inputReader.close();
    }
}