QuickSelect

import random

def quickselect(arr, k):
    """
    Returns the k-th smallest element of list within arr.
    """
    if len(arr) == 1:
        return arr[0]

    pivot = random.choice(arr)

    lows = [el for el in arr if el < pivot]
    highs = [el for el in arr if el > pivot]
    pivots = [el for el in arr if el == pivot]

    if k < len(lows):
        return quickselect(lows, k)
    elif k < len(lows) + len(pivots):
        return pivots[0]
    else:
        return quickselect(highs, k - len(lows) - len(pivots))

# Example usage:
arr = [3, 2, 1, 5, 4]
k = 2  # looking for the 3rd smallest element, index 2
print(quickselect(arr, k))  # Output: 3