🔄 Move All Negative Elements to End – Python

Hi All,

Today I solved an interesting array problem: Move all negative elements to the end of the array while maintaining order.

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📌 Problem Statement

Given an array containing both positive and negative integers, move all negative elements to the end.

⚠️ Conditions:

• Maintain the order of elements
• Perform operation in-place
• Do not return a new array

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🔍 Examples

Example 1:

arr = [1, -1, 3, 2, -7, -5, 11, 6]

Output:

[1, 3, 2, 11, 6, -1, -7, -5]

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Example 2:

arr = [-5, 7, -3, -4, 9, 10, -1, 11]

Output:

[7, 9, 10, 11, -5, -3, -4, -1]

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💡 Approach

🔹 Method: Using Extra List (Stable Order)

👉 Since we must maintain order, we:

1. Store all positive elements
2. Store all negative elements
3. Combine them back into original array

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💻 Python Code

def move_negatives(arr):
    positives = []
    negatives = []

    for num in arr:
        if num >= 0:
            positives.append(num)
        else:
            negatives.append(num)

    # Modify original array (in-place)
    arr[:] = positives + negatives

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🔍 Dry Run

For:

arr = [1, -1, 3, -2]

Steps:

• positives → [1, 3]
• negatives → [-1, -2]

Final array:

[1, 3, -1, -2]

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🖥️ Sample Output

Input: [1, -1, 3, 2, -7, -5, 11, 6]
Output: [1, 3, 2, 11, 6, -1, -7, -5]

Input: [-5, 7, -3, -4, 9, 10, -1, 11]
Output: [7, 9, 10, 11, -5, -3, -4, -1]

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⚡ Complexity Analysis

• Time Complexity: O(n) ✅
• Space Complexity: O(n) ✅

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🧠 Why not two-pointer swap?

👉 Normal swapping:

• Changes order ❌
• Not allowed in this problem

👉 This problem needs stable arrangement

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✅ Conclusion

This problem helped me understand:

• Stable partitioning
• Maintaining order in arrays
• In-place modification using slicing

🚀 Useful concept for real-world data processing!

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