Sort in specific order

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Problem Overview

Sort array where odd numbers come first (sorted descending) then even numbers (sorted ascending).

Intuition

Sort array where odd numbers come first (sorted descending) then even numbers (sorted ascending).

Algorithm

1Separate odd and even. Sort odds descending, evens ascending. Concatenate odds + evens.

Common Pitfalls

• Partition then sort each group. O(n log n). Specific ordering depends on problem variant.

Sort in specific order.java

Java

// Approach: Partition into two groups (odd/even, positive/negative, etc.) then arrange per specification.
// Time: O(n log n) Space: O(n)
import java.util.*;

class Solution {
    public void sortIt(int[] arr) {
        int n = arr.length;
        List<Integer> odd = new ArrayList<>();
        List<Integer> even = new ArrayList<>();

        for (int i = 0; i < n; i++) {
            if (arr[i] % 2 == 0)
                even.add(arr[i]);
            else
                odd.add(arr[i]);
        }

        Collections.sort(odd);
        Collections.reverse(odd);
        Collections.sort(even);
        int i = 0;
        int j = 0;
        
        while (i < odd.size()) {
            arr[j] = odd.get(i);
            i++;
            j++;
        }
        // j++;
        int k = 0;
        while (k < even.size()) {
            arr[j] = even.get(k);
            k++;
            j++;
        }
    }
}

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