寻找第k个最小元素

Question

I followed the algorithm in the book for this problem. when I print result it is not correct. the algorithm is exactly as in the book my code

import math

def quickSelect(A, k):
  m = A[math.floor(len(A)/2)]
  L = [i for i in A if i  < m]
  E = [i for i in A if i == m]
  G = [i for i in A if i  > m]
  print(L)
  print(E)
  print(G)

  if k <= len(L): 
    return quickSelect(L, k)
  elif k <= (len(E) + len(G)):
    return m
  else:
    return quickSelect(G, k- len(L) - len(E))

result = quickSelect([7, 14, 10, 12, 2, 11, 29, 3, 4], 4)

print(result)

Answer 1

These statements:

L = [i for i in A if i < m]      # less than m
E = [i for i in A if i == m]     # equal to m
G = [i for i in A if i > m]      # greater than m

partition the array into three ranges:

|   L1 L2 L3 L4   |   E1   |   G1 G2 G3
|                 |        |
0                 |        |
                 len(L)    |
                           len(L) + len(E)

Your condition for the second range,

elif k <= (len(L) + len(G)):
    return m

is wrong. It should be:

elif k <= (len(L) + len(E)):
    return m

Node: Instead of using floating-point math, you can just calculate m with Python's integer division: m = A[len(A) // 2]