Sort in specific order

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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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