学步园

Problem 1.5: Write a method to replace all spaces in a string with '%20'.

The first solution flashed into my mind takes O(n), and it's similar to the one on the answer page.

import string
def replace_space(str):
    # Count the number of spaces
    space_count = 0
    for i in range(0, len(str)):
        if str[i] == ' ':
            space_count = space_count + 1
    
    # Allocate a new string
    new_str = [' ' for i in range(0, len(str) + 2*space_count)]
    
    #Copy the original string into new string,
    # while replacing spaces with "%20"
    j = 0
    for i in range(0, len(str)):
        if str[i] == ' ':
            new_str[j] = '%'
            new_str[j+1] = '2'
            new_str[j+2] = '0'
            j = j + 3
        else:
            new_str[j] = str[i]
            j = j + 1
    return string.join(new_str, '')

One thing to note is the string.join() function, which takes the connector as the second parameter and the default connector is a space.