344. Reverse String

Write a functon that reverses an input string. The string is provided as a character array s.

You must modify the input array in-place and use only O(1) extra memory.

Example 1:

Input: s = ["h","e","l","l","o"]
Output: ["o","l","l","e","h"]

Solution: Use a two-pointer approach. Initialize pointers at both ends of the array and swap characters while moving towards the center.

Implementation:

public class ReverseString {
    public void reverse(char[] chars) {
        int start = 0;
        int end = chars.length - 1;
        while (start < end) {
            char tmp = chars[end];
            chars[end] = chars[start];
            chars[start] = tmp;
            start++;
            end--;
        }
    }
}

541. Reverse String II

Given a string s and an integer k, reverse the first k characters for every 2k characters counting from the start of the string.

If there are fewer than k characters left, reverse all of them. If there are between k and 2k characters left, reverse the first k characters and leave the rest as is.

Example 1:

Input: s = "abcdefg", k = 2
Output: "bacdfeg"

Solution: Process the string in segments of length 2k. For each segment, reverse its first k characters. When dealing with strings in fixed intervals, consider adjusting the loop increment accordingly.

Implementation:

public class ReverseStringII {
    public String reverseSegments(String str, int k) {
        char[] arr = str.toCharArray();
        for (int idx = 0; idx < arr.length; idx += 2 * k) {
            int left = idx;
            int right = Math.min(arr.length - 1, idx + k - 1);
            while (left < right) {
                char temp = arr[left];
                arr[left] = arr[right];
                arr[right] = temp;
                left++;
                right--;
            }
        }
        return new String(arr);
    }
}

54. Replace Digits

Given a string s consisting of lowercase letters and digits, write a function that keeps letters unchanged and replaces each digit with the word "number".

Example: For input "a1b2c3", the output should be "anumberbnumbercnumber".

Solution: Convert the string to a mutable sequence like StringBuilder to efficiently append characters or replacement strings. Use Character.isDigit() to identify numeric characters.

Implementation:

import java.util.Scanner;

public class DigitReplacer {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        String original = input.nextLine();
        StringBuilder result = new StringBuilder();
        for (int pos = 0; pos < original.length(); pos++) {
            char current = original.charAt(pos);
            if (Character.isDigit(current)) {
                result.append("number");
            } else {
                result.append(current);
            }
        }
        System.out.println(result.toString());
    }
}