String Reversal Approaches in DSA Typescript - Build from Scratch

String Reversal Approaches

📖 Scenario: Imagine you are building a simple tool that helps users reverse words or sentences. This is useful in games, puzzles, or text processing apps.

🎯 Goal: You will create a program that takes a string and reverses it using different methods step-by-step.

📋 What You'll Learn

Create a string variable with a specific value

Create a helper variable to hold the reversed string

Use a for loop to reverse the string manually

Print the reversed string

💡 Why This Matters

🌍 Real World

Reversing strings is useful in text processing, puzzles, and coding challenges.

💼 Career

Understanding string manipulation is a basic skill for software developers working with user input and data transformation.

1

Create the original string

Create a string variable called original and set it to the value "hello".

DSA Typescript

// Your code here

Hint

Use const to create a string variable named original and assign it the text "hello".

2

Create a variable to hold the reversed string

Create a string variable called reversed and set it to an empty string "".

DSA Typescript

const original: string = "hello";
// Your code here

Hint

Use let to create a variable named reversed and start it as an empty string.

3

Reverse the string using a for loop

Use a for loop with a variable i starting from original.length - 1 down to 0. Inside the loop, add original[i] to reversed.

DSA Typescript

const original: string = "hello";
let reversed: string = "";
// Your code here

Hint

Start the loop from the last character index and move backward to the first. Add each character to reversed.

4

Print the reversed string

Use console.log to print the value of reversed.

DSA Typescript

const original: string = "hello";
let reversed: string = "";
for (let i = original.length - 1; i >= 0; i--) {
    reversed += original[i];
}
// Your code here

Hint

Use console.log(reversed) to show the reversed string on the screen.

Practice

1. What does the split('') method do when applied to a string in TypeScript?

easy

A. It reverses the string directly.

B. It converts the string into an array of individual characters.

C. It joins the string elements into one string.

D. It removes all spaces from the string.

Solution

1. Step 1: Understand the split method

   The split('') method breaks a string into an array of characters by splitting at every empty string between characters.

2. Step 2: Effect on string

   For example, "hello".split('') becomes ["h", "e", "l", "l", "o"].

3. Final Answer:

   It converts the string into an array of individual characters. -> Option B

4. Quick Check:

   split('') = array of chars [OK]

Hint: split('') breaks string into chars array [OK]

Common Mistakes:

• Thinking split('') reverses the string
• Confusing split('') with join()
• Assuming split('') removes spaces

2. Which of the following is the correct syntax to reverse a string str using built-in methods in TypeScript?

easy

A. str.split('').reverse().join('')

B. str.reverse().join('')

C. reverse(str).split('')

D. str.join('').reverse()

Solution

1. Step 1: Split string into array

   Use str.split('') to convert string into array of characters.

2. Step 2: Reverse and join

   Use reverse() on array, then join('') to form reversed string.

3. Final Answer:

   str.split('').reverse().join('') -> Option A

4. Quick Check:

   split + reverse + join = reversed string [OK]

Hint: Split, reverse array, then join to reverse string [OK]

Common Mistakes:

• Calling reverse() directly on string
• Using join() before reverse()
• Using reverse() as a standalone function

3. What will be the output of the following TypeScript code?

const str = "abcde";
const reversed = str.split('').reverse().join('');
console.log(reversed);

medium

A. "edcbe"

B. "abcde"

C. ""

D. "edcba"

Solution

1. Step 1: Split string into array

   "abcde".split('') becomes ["a", "b", "c", "d", "e"].

2. Step 2: Reverse and join array

   Reversing gives ["e", "d", "c", "b", "a"], joining results in "edcba".

3. Final Answer:

   "edcba" -> Option D

4. Quick Check:

   split + reverse + join = "edcba" [OK]

Hint: Split, reverse, join outputs reversed string [OK]

Common Mistakes:

• Expecting original string output
• Mixing up reversed characters
• Forgetting to join after reverse

4. Identify the error in this TypeScript function intended to reverse a string:

function reverseString(s: string): string {
  return s.reverse();
}

medium

A. Strings do not have a reverse() method directly.

B. The function should return void, not string.

C. The parameter should be an array, not a string.

D. The function is missing a return statement.

Solution

1. Step 1: Check string methods

   Strings in TypeScript do not have a reverse() method directly; only arrays do.

2. Step 2: Correct approach

   Convert string to array with split(''), reverse array, then join back.

3. Final Answer:

   Strings do not have a reverse() method directly. -> Option A

4. Quick Check:

   reverse() works on arrays, not strings [OK]

Hint: reverse() is for arrays, not strings [OK]

Common Mistakes:

• Trying to call reverse() on string directly
• Ignoring need to split string first
• Assuming reverse() returns void

5. Given the function below, what will be the output when calling reverseString('12345')?

function reverseString(s: string): string {
  let reversed = '';
  for (let i = s.length - 1; i >= 0; i--) {
    reversed += s[i];
  }
  return reversed;
}

hard

A. "5432"

B. "12345"

C. "54321"

D. ""

Solution

1. Step 1: Loop from end to start

   The loop starts at index 4 (last char '5') and moves backward to 0.

2. Step 2: Build reversed string

   Characters are appended in reverse order: '5', '4', '3', '2', '1'.

3. Final Answer:

   "54321" -> Option C

4. Quick Check:

   Loop backward concatenates reversed string [OK]

Hint: Loop backward to build reversed string [OK]

Common Mistakes:

• Looping forward instead of backward
• Off-by-one errors in loop index
• Not initializing reversed string properly