🔁 Reverse an Array

Reversing an array is one of the most fundamental problems in programming.
It’s simple to understand but extremely useful in real-world applications and coding interviews.

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📌 What Does "Reverse an Array" Mean?

Reversing an array means rearranging its elements so that:

• The first element becomes the last
• The second element becomes the second-last
• And so on...

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🔍 Examples

Example 1:

Input:  [1, 4, 3, 2, 6, 5]
Output: [5, 6, 2, 3, 4, 1]

Example 2:

Input:  [4, 5, 1, 2]
Output: [2, 1, 5, 4]

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🧠 Concept Behind Reversing

We use the two-pointer technique:

• One pointer starts at the beginning
• One pointer starts at the end
• Swap elements and move inward

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🔄 Approach 1: Two-Pointer Method (Most Efficient)

Step-by-Step:

1. Set start = 0
2. Set end = n - 1
3. Swap elements at start and end
4. Move:

• start = start + 1
• end = end - 1
  1. Repeat until start < end

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💻 Python Code:

def reverse_array(arr):
    start = 0
    end = len(arr) - 1

    while start < end:
        arr[start], arr[end] = arr[end], arr[start]
        start += 1
        end -= 1

    return arr

print(reverse_array([1, 4, 3, 2, 6, 5]))

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🧾 Dry Run (Step-by-Step)

For:

arr = [1, 4, 3, 2, 6, 5]

Step	Start	End	Array After Swap
1	0	5	[5, 4, 3, 2, 6, 1]
2	1	4	[5, 6, 3, 2, 4, 1]
3	2	3	[5, 6, 2, 3, 4, 1]

Loop stops when start >= end

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⚡ Time and Space Complexity

• Time Complexity: O(n)
• Space Complexity: O(1) (in-place)

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🔁 Approach 2: Using Loop (Extra Space)

def reverse_array(arr):
    reversed_arr = []
    for i in range(len(arr) - 1, -1, -1):
        reversed_arr.append(arr[i])
    return reversed_arr

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⚡ Approach 3: Python Shortcut

arr = [1, 4, 3, 2, 6, 5]
arr.reverse()
print(arr)

or

print(arr[::-1])

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🧩 Where Is This Used?

• Data processing
• Algorithm design
• Palindrome problems
• Interview questions

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🏁 Conclusion

Reversing an array is a basic yet powerful operation.
The two-pointer approach is the most efficient and commonly used method.

Once you understand this, you can move on to more advanced problems like:

• Array rotation
• String reversal
• Palindrome checking

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💬 What’s your favorite way to reverse an array? Let me know in the comments!