Find the kth largest element in an unsorted array using Quickselect?

Short answer: public int FindKthLargest(int[] nums, int k) { return QuickSelect(nums, 0, nums.Length - 1, nums.Length - k); } private int QuickSelect(int[] nums, int left, int right, int kSmallest) { if (left == right) return nums[left]; int pivotIndex = Partition(nums, left, right); if (kSmallest == pivotIndex) return nums[kSmallest]; else if (kSmallest < pivotIndex) return QuickSelect(nums, left, pivotIndex - 1, kSmallest);…

Explain a bit more

else return QuickSelect(nums, pivotIndex + 1, right, kSmallest); } private int Partition(int[] nums, int left, int right) { int pivot = nums[right]; int i = left; for (int j = left; j < right; j++) { if (nums[j] <= pivot) { Swap(nums, i, j); i++; } } Swap(nums, i, right); return i; } private void Swap(int[] nums, int i, int j) { int temp = nums[i]; nums[i] = nums[j]; nums[j] = temp; } Follow on: Explanation: Quickselect partitions the array like Quicksort and recursively searches for the kth smallest element. Here, nums.Length - k gives the kth largest.

Example code

public int FindKthLargest(int[] nums, int k) {
return QuickSelect(nums, 0, nums.Length - 1, nums.Length - k);
}
private int QuickSelect(int[] nums, int left, int right, int kSmallest) { if (left == right) return nums[left];
int pivotIndex = Partition(nums, left, right);
if (kSmallest == pivotIndex)
return nums[kSmallest]; else if (kSmallest < pivotIndex) return QuickSelect(nums, left, pivotIndex - 1, kSmallest); else return QuickSelect(nums, pivotIndex + 1, right, kSmallest);
}
private int Partition(int[] nums, int left, int right) {
int pivot = nums[right];
int i = left;
for (int j = left; j < right; j++) {
if (nums[j] <= pivot) { Swap(nums, i, j); i++; }
} Swap(nums, i, right); return i;
}
private void Swap(int[] nums, int i, int j) {
int temp = nums[i];
nums[i] = nums[j];
nums[j] = temp;
} Follow on: Explanation: Quickselect partitions the array like Quicksort and recursively searches for the kth smallest element. Here, nums.Length - k gives the kth largest.

Real-world example (ShopNest)

In coding rounds, state complexity aloud, write a clear ShopNest-flavored example (orders, carts), then handle edge cases (empty list, null, overflow).

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.