Interviews: Reversing a String

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Question:

Simply reverse a string

Solutions

To start out, I went with a needlessly complicated, naive approach:

def revStrLoop(input):
	outList = [0]*len(input)
	for i, char in enumerate(input):
		outList[len(input)-i-1] = char
	return ''.join(outList)

This is nice, I suppose, if you’re looking to show off your ability to iterate lists, but otherwise it’s just a bulky mess.

Fortunately, Python’s list implements reversed() so we can use the builtin reversed call:

''.join(reversed(list(input)))

This is a nice almost functional design. One obvious problem still remains: we’re still looking at the string post conversion to list. Handling the string as a list is nice when you’re doing more fiddly things as strings are immutable. However, for simple things like this, we can use Python’s builtin slicing:

input[::-1]

The slice syntax is: [start:end:step]. Negative steps indicate descending ordering. Python is rather nice when it comes to defaults. If the step is negative, the default start is the end of the list/array/string/… whereas if the step is positive, the default is at the start.

Results

Just to make sure the slicing is the best from a speed standpoint as well as clean syntactically, I ran all three through this function:

def testTiming(func, string):
    initT = time.process_time()
    for _ in range(10000):
        func(string)
    return time.process_time() - initT

Using slicing improved by a factor of about 5 over the join approach and a factor of about 14 over the iteration approach.

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