Python/data_structures/stacks/next_greater_element.py

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from __future__ import annotations
arr = [-10, -5, 0, 5, 5.1, 11, 13, 21, 3, 4, -21, -10, -5, -1, 0]
expect = [-5, 0, 5, 5.1, 11, 13, 21, -1, 4, -1, -10, -5, -1, 0, -1]
def next_greatest_element_slow(arr: list[float]) -> list[float]:
"""
Get the Next Greatest Element (NGE) for all elements in a list.
Maximum element present after the current one which is also greater than the
current one.
>>> next_greatest_element_slow(arr) == expect
True
"""
result = []
arr_size = len(arr)
for i in range(arr_size):
next: float = -1
for j in range(i + 1, arr_size):
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if arr[i] < arr[j]:
next = arr[j]
break
result.append(next)
return result
def next_greatest_element_fast(arr: list[float]) -> list[float]:
"""
Like next_greatest_element_slow() but changes the loops to use
enumerate() instead of range(len()) for the outer loop and
for in a slice of arr for the inner loop.
>>> next_greatest_element_fast(arr) == expect
True
"""
result = []
for i, outer in enumerate(arr):
next: float = -1
for inner in arr[i + 1 :]:
if outer < inner:
next = inner
break
result.append(next)
return result
def next_greatest_element(arr: list[float]) -> list[float]:
"""
Get the Next Greatest Element (NGE) for all elements in a list.
Maximum element present after the current one which is also greater than the
current one.
A naive way to solve this is to take two loops and check for the next bigger
number but that will make the time complexity as O(n^2). The better way to solve
this would be to use a stack to keep track of maximum number giving a linear time
solution.
>>> next_greatest_element(arr) == expect
True
"""
arr_size = len(arr)
stack: list[float] = []
result: list[float] = [-1] * arr_size
for index in reversed(range(arr_size)):
if stack:
while stack[-1] <= arr[index]:
stack.pop()
if not stack:
break
if stack:
result[index] = stack[-1]
stack.append(arr[index])
return result
if __name__ == "__main__":
from doctest import testmod
from timeit import timeit
testmod()
print(next_greatest_element_slow(arr))
print(next_greatest_element_fast(arr))
print(next_greatest_element(arr))
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setup = (
"from __main__ import arr, next_greatest_element_slow, "
"next_greatest_element_fast, next_greatest_element"
)
print(
"next_greatest_element_slow():",
timeit("next_greatest_element_slow(arr)", setup=setup),
)
print(
"next_greatest_element_fast():",
timeit("next_greatest_element_fast(arr)", setup=setup),
)
print(
" next_greatest_element():",
timeit("next_greatest_element(arr)", setup=setup),
)