This notebook was prepared by [Donne Martin](http://donnemartin.com). Source and license info is on [GitHub](https://github.com/donnemartin/interactive-coding-challenges).

Solution Notebook

Problem: Implement a function to reverse a string (a list of characters), in-place.

Constraints
Test Cases
Algorithm
Code
Pythonic-Code
Unit Test
Bonus C Algorithm
Bonus C Code

Constraints

Can I assume the string is ASCII?
- Yes
- Note: Unicode strings could require special handling depending on your language
Since we need to do this in-place, it seems we cannot use the slice operator or the reversed function?
- Correct
Since Python string are immutable, can I use a list of characters instead?
- Yes

Test Cases

None -> None
[''] -> ['']
['f', 'o', 'o', ' ', 'b', 'a', 'r'] -> ['r', 'a', 'b', ' ', 'o', 'o', 'f']

Algorithm

Since Python strings are immutable, we'll use a list of chars instead to exercise in-place string manipulation as you would get with a C string.

Iterate len(string)/2 times, starting with i = 0:
- Swap i and len(string) - 1 - i
- Increment i

Complexity:

Time: O(n)
Space: O(1)

Note:

You could use a byte array instead of a list to do in-place string operations

Code



In [1]:

    
from __future__ import division


def list_of_chars(chars):
    if chars is None:
        return None
    size = len(chars)
    for i in range(size//2):
        chars[i], chars[size-1-i] = \
            chars[size-1-i], chars[i]
    return chars

Pythonic-Code

This question has an artificial constraint that prevented the use of the slice operator and the reversed method. For completeness, the solutions for these are provided below. Note these solutions are not in-place.



In [2]:

    
def reverse_string_alt(string):
    if string is None:
        return None
    return string[::-1]


def reverse_string_alt2(string):
    if string is None:
        return None
    return ''.join(reversed(string))

Unit Test



In [3]:

    
%%writefile test_reverse_string.py
from nose.tools import assert_equal


class TestReverse(object):

    def test_reverse(self):
        assert_equal(list_of_chars(None), None)
        assert_equal(list_of_chars(['']), [''])
        assert_equal(list_of_chars(
            ['f', 'o', 'o', ' ', 'b', 'a', 'r']),
            ['r', 'a', 'b', ' ', 'o', 'o', 'f'])
        print('Success: test_reverse')


def main():
    test = TestReverse()
    test.test_reverse()


if __name__ == '__main__':
    main()









    



Overwriting test_reverse_string.py



In [4]:

    
%run -i test_reverse_string.py









    



Success: test_reverse

C Algorithm

This is a classic problem in C/C++

We'll want to keep two pointers:

i is a pointer to the first char
j is a pointer to the last char

To get a pointer to the last char, we need to loop through all characters, take note of null terminator.

while i < j
- swap i and j

Complexity:

Time: O(n)
Space: In-place

Note:

Instead of using i, you can use str instead, although this might not be as intuitive.

C Code



In [ ]:

    
# %load reverse_string.cpp
#include <stdio.h>

void Reverse(char* str) {
    if (str) {
        char* i = str;	// first letter
        char* j = str;	// last letter
        
        // find the end of the string
        while (*j) {
            j++;
        }
        
        // don't point to the null terminator
        j--;
        
        char tmp;
        
        // swap chars to reverse the string
        while (i < j) {
            tmp = *i;
            *i++ = *j;
            *j-- = tmp;
        }
    }
}

int main() {
    char test0[] = "";
    char test1[] = "foo";
    Reverse(NULL);
    Reverse(test0);
    Reverse(test1);
    printf("%s \n", test0);
    printf("%s \n", test1);
    return 0;
}