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reverse_array.py
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57 lines (44 loc) · 1.42 KB
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'''
Reverse array
Reverse an array, in constant space and linear time complexity.
Input: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
Output: [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]
=========================================
Reverse the whole array by swapping pair letters in-place (first with last, second with second from the end, etc).
Exist 2 more "Pythonic" ways of reversing arrays/strings (but not in-place, they're creating a new list):
- reversed_arr = reversed(arr)
- reversed_arr = arr[::-1]
But I wanted to show how to implement a reverse algorithm step by step so someone will know how to implement it in other languages.
Time Complexity: O(N)
Space Complexity: O(1)
'''
############
# Solution #
############
def reverse_arr(arr):
start = 0
end = len(arr) - 1
while start < end:
# reverse the array from the start index to the end index by
# swaping each element with the pair from the other part of the array
swap(arr, start, end)
start += 1
end -= 1
return arr
def swap(arr, i, j):
# swapping two elements from a same array
arr[i], arr[j] = arr[j], arr[i]
'''same as
temp = arr[i]
arr[i] = arr[j]
arr[j] = temp
'''
###########
# Testing #
###########
# Test 1
# Correct result => [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]
print(reverse_arr([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]))
# Test 2
# Correct result => [5, 4, 3, 2, 1]
print(reverse_arr([1, 2, 3, 4, 5]))