Reorganize The Array

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Problem Overview

Rearrange array so all negative elements appear before positive.

See our study guide for structured GFG and LeetCode practice.

Intuition

Rearrange array so all negative elements appear before positive. Maintain relative order (stable).

Algorithm

1Two passes: collect negatives maintaining order, then positives. Concatenate.

Common Pitfalls

• Stable partition. O(n) with extra space. In-place stable partition is O(n log n).

Reorganize The Array.java

Java

// Approach: Sort, then split at midpoint and interleave elements from both halves.
// Time: O(n log n) Space: O(n)
class Solution {
    public List<Integer> rearrange(List<Integer> arr) {
        int n = arr.size();
        List<Integer> ans = new ArrayList<Integer>();

        for (int i = 0; i < arr.size(); i++)
            ans.add(-1);

        for (int ele : arr) {
            if (ele >= 0 && ele < arr.size())
                ans.set(ele, ele);
        }

        return ans;
    }
}

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