Reverse String

Reverse a string, represented as an array of characters s, in place without using extra memory.

Here’s a step-by-step guide to solve this problem:

  1. Use Two Pointers: Initialize two pointers, one at the beginning (left) and the other at the end (right) of the array.
  2. Swap the Characters: Swap the characters at the left and right pointers.
  3. Move the Pointers: Increment the left pointer and decrement the right pointer.
  4. Repeat Until Pointers Meet: Continue steps 2 and 3 until the left pointer is no longer less than the right pointer.

Here’s a code example that implements these steps:

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class Solution:
    def reverseString(self, s: List[str]) -> None:
        left, right = 0, len(s) - 1
        
        while left < right:
            s[left], s[right] = s[right], s[left]
            left += 1
            right -= 1

Explanation:

  • The code initializes two pointers left and right at the beginning and end of the array, respectively.
  • Inside a loop, the code swaps the elements at the left and right pointers and then increments the left pointer and decrements the right pointer.
  • This continues until the pointers meet, resulting in the reversal of the array.

This solution adheres to the constraint of using O(1) extra memory and reverses the array in place.

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# @param {Character[]} s
# @return {Void} Do not return anything, modify s in-place instead.
def reverse_string(s)
    i = 0
    j = s.size - 1

    while i < j
        s[i], s[j] = s[j], s[i]
        i += 1
        j -= 1
    end
    return s
end

Prerequisite LeetCode Problems

“Reverse String” is one of the basic problems which helps you understand the fundamental concept of array manipulation and two-pointer technique. It’s hard to find simpler problems before this, however, you can look at these for practice and understanding:

  1. “125. Valid Palindrome”: After understanding reversing of string, validating a palindrome string would be a good problem to solve next as it extends the concept of reversing a string.
  2. “283. Move Zeroes”: This problem helps you understand the concept of in-place array manipulations.
  3. “27. Remove Element”: This problem is similar to “Move Zeroes” and also helps understand in-place array manipulation.
  4. “26. Remove Duplicates from Sorted Array”: This problem introduces the concept of dealing with duplicates and also uses in-place array manipulations.

These problems would help you get more comfortable with array manipulations and two-pointer techniques which are often used in many other problems.

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class Solution:
    def reverseString(self, s: List[str]) -> None:
        # Two Pointer
        left, right =0, len(s) - 1

        while left < right:
            s[left], s[right] = s[right], s[left]
            left += 1
            right -= 1

Problem Classification

Based on the terminology used in the problem statement, we can classify this problem as:

  1. Array Manipulation: The problem requires you to manipulate an input array in-place.

  2. String Processing: It is related to manipulating and understanding the properties of strings, represented as an array of characters here.

  3. In-place Algorithm: It explicitly asks for an in-place solution, which means the algorithm is required to transform input using no auxiliary data structure.

  4. Space Complexity: The problem explicitly imposes an O(1) extra memory constraint, which is about managing and optimizing the space usage of the algorithm.

  5. Two-pointer Technique: Although not explicitly stated, the common way to solve such problems is to use the two-pointer technique where one pointer starts from the beginning and the other from the end.

Note: The last point is about the typical solution approach rather than the problem itself, but it’s often useful to identify these patterns when classifying problems as it helps in deciding a strategy to solve them.

Language Agnostic Coding Drills

  1. Understanding Arrays: Understand how arrays work, how to access the elements of an array using indexing, and how to modify the elements of an array.

  2. Understanding Strings and Characters: Understanding that strings can be thought of as arrays of characters. Knowing how to access and swap individual characters is key to reversing a string.

  3. In-place modification of an array: This requires an understanding that the elements of an array can be changed or swapped without needing to create a new array.

  4. Two-pointer technique: This is a commonly used technique to traverse an array from two ends towards the center. Understand how to initialize two pointers at both ends of the array and move them towards each other.

  5. Loop control and termination: Understand how to set up a while loop that continues as long as the left pointer is less than the right pointer. Know when the loop should terminate.

  6. Swapping elements in an array: In each loop iteration, understand how to swap the elements at the positions pointed to by the left and right pointers.

  7. Updating pointers: Understand how to increment the left pointer and decrement the right pointer after each swap.

By learning these concepts separately and then combining them, you can understand and implement the provided solution for reversing a string.

Here’s a step-by-step guide of the problem-solving approach:

  1. Read and understand the problem. We need to reverse a string by modifying the input array in-place.

  2. Identify the key concepts or techniques that apply to this problem. In this case, the key technique is the two-pointer technique.

  3. Start by initializing two pointers: one at the start of the string (left), and one at the end of the string (right).

  4. Set up a while loop that continues as long as the left pointer is less than the right pointer.

  5. Within the loop, swap the characters at the positions pointed to by the left and right pointers.

  6. After swapping, increment the left pointer and decrement the right pointer to move them towards each other.

  7. The loop will automatically terminate when all characters have been swapped, i.e., when the left pointer is no longer less than the right pointer. At this point, the string has been reversed in-place.

  8. Since the operation was done in-place, there is no need for a return statement. The original string array has now been reversed.

Targeted Drills in Python

  1. Understanding Arrays: Create an array, access elements, and modify them.
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# Create an array
arr = ['h', 'e', 'l', 'l', 'o']

# Access and print the first element
print(arr[0])

# Modify the last element
arr[-1] = 'a'
print(arr)
  1. Understanding Strings and Characters: Create a string, convert it into an array of characters.
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# Create a string
str = "hello"

# Convert the string into a list of characters
char_list = list(str)
print(char_list)
  1. In-place modification of an array: Swap the first and last elements of an array.
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# Swap the first and last elements
arr[0], arr[-1] = arr[-1], arr[0]
print(arr)
  1. Two-pointer technique: Initialize two pointers and use them to traverse an array.
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# Initialize two pointers
left = 0
right = len(arr) - 1

# Print elements from both ends
while left <= right:
    print(arr[left], arr[right])
    left += 1
    right -= 1
  1. Loop control and termination: Add a condition to the loop to terminate it when the pointers cross.
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# Reset the pointers
left = 0
right = len(arr) - 1

# Print elements and stop when pointers cross
while left < right:
    print(arr[left], arr[right])
    left += 1
    right -= 1
  1. Swapping elements in an array: Swap elements at the positions pointed to by two pointers.
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# Reset the pointers
left = 0
right = len(arr) - 1

# Swap elements and print the array
while left < right:
    arr[left], arr[right] = arr[right], arr[left]
    left += 1
    right -= 1
print(arr)

After these drills, you should be able to combine all these concepts to reverse a string array in-place by swapping elements at positions pointed to by two pointers moving towards each other.

Coding Skill Exercise #17

Reverse String

Implement a method reverse that reverses a given string.

Knowledge Gap Finder

If you are unable to code the solution, answer the following questions and reply to this email to get customized lesson.

Was the problem statement clear to you when you read it? What did you think of doing first? Were you reminded of a construct in general or a general structure of solution that you thought would be useful? Have you previously seen problems that resemble this one? Did you feel stuck at any point while working on this problem? What did you choose as your test case? Do you think you’ve covered all possible scenarios with your tests? What program design techniques did you apply to solve this problem? Are there any constructs of the programming language that you find difficult or confusing to use? What issues make programming constructs difficult to use? For example, the keyword used, the syntax, the examples, the documentation for the construct, etc.

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