from random import randint
def swap(array:[], index1, index2):
array[index1], array[index2] = array[index2], array[index1]
def partition(array:[], startIndex:int, endIndex:int) -> int:
pivot = array[endIndex]
lastLessorIndex = startIndex - 1
for i in range(startIndex, endIndex):
if array[i] <= pivot:
lastLessorIndex += 1
swap(array, i, lastLessorIndex)
lastLessorIndex += 1
swap(array, endIndex, lastLessorIndex)
return lastLessorIndex
def randomizedPartition(array:[], startIndex:int, endIndex:int) -> int:
swap(array, endIndex, randint(startIndex, endIndex))
return partition(array, startIndex, endIndex)
def randomizedSelect(array:[], startIndex:int, endIndex:int, targetOrder:int) -> int:
# targetOrder that begins with 0 serves more like an index.
if startIndex == endIndex:
return array[startIndex]
currentIndex = randomizedPartition(array, startIndex, endIndex)
targetIndex = startIndex + targetOrder
if currentIndex == targetIndex:
return array[currentIndex]
elif currentIndex < targetIndex:
return randomizedSelect(array, currentIndex+1, endIndex, \
targetIndex-currentIndex-1)
else:
return randomizedSelect(array, startIndex, currentIndex-1, targetOrder)
Randomized Select. Expected Θ(n).
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