# Write a method to replace all spaces in a string with '%20'.

def urlifyString(str):
    res = []
    start = False
    str = str.strip() # Removes white space from the beginning and end of the string
    for char in reversed(str):
        if(char == ' '):
            res += '02%'
        else: res += char
    res = ''.join(res[::-1])
    return res

print urlifyString("       Mr John Smith           ")

# O(n)