Array Reversal: O(n / 2)¶

In [1]:

arr = [1, 2, 3, 4, 5, 6]
arr

Out[1]:

[1, 2, 3, 4, 5, 6]

In [2]:

arr.reverse()    # Inplace
arr

Out[2]:

[6, 5, 4, 3, 2, 1]

In [3]:

arr[::-1]    # Not Inplace

Out[3]:

[1, 2, 3, 4, 5, 6]

In [4]:

arr

Out[4]:

[6, 5, 4, 3, 2, 1]

Iterative¶

In [5]:

def reverseArr(arr):
    start, end = 0, len(arr) - 1
    
    while start < end:
        arr[start], arr[end] = arr[end], arr[start]
        
        start += 1
        end -= 1

In [6]:

reverseArr(arr)
arr

Out[6]:

[1, 2, 3, 4, 5, 6]

Recursive¶

In [7]:

def recursiveReverseArr(arr, start, end):
    if start >= end:
        return
    
    arr[start], arr[end] = arr[end], arr[start]
    
    recursiveReverseArr(arr, start + 1, end - 1)

In [8]:

recursiveReverseArr(arr, 0, len(arr) - 1)
arr

Out[8]:

[6, 5, 4, 3, 2, 1]