Python code to move spaces to front of string in single traversal

Given a string that has set of words and spaces, write a program to move all spaces to front of string, by traversing the string only once. Examples:

Input  : str = "geeks for geeks"
Output : str = "  geeksforgeeks"

Input  : str = "move these spaces to beginning"
Output : str = "    movethesespacestobeginning"
There were four space characters in input,
all of them should be shifted in front. 

This problem has existing solution, please refer Move spaces to front of string in single traversal link. We will solve this problem quickly in Python using List Comprehension. 

Approach 1:

1. Traverse input string and create a string without any space character using list comprehension.
2. Now to know how many space characters were there in original string just take a difference of length of original string and new string.
3. Now create another string and append space characters at the beginning.

Implementation

Python3

# Function to move spaces to front of string
# in single traversal in Python

def moveSpaces(input):

    # Traverse string to create string without spaces
    noSpaces = [ch for ch in input if ch!=' ']

    # calculate number of spaces
    space= len(input) - len(noSpaces)

    # create result string with spaces
    result = ' '*space

    # concatenate spaces with string having no spaces
    result = '"'+result + ''.join(noSpaces)+'"'
    print (result)

# Driver program
if __name__ == "__main__":
    input = 'geeks for geeks'
    moveSpaces(input)

Output:

"  geeksforgeeks"

Time Complexity: O(n)
Auxiliary Space: O(n)

Approach 2 : Using count() and replace() method

Implementation

Python3

# Function to move spaces to front of string
# in single traversal in Python

def moveSpaces(input):
    c=input.count(' ')
    input=input.replace(' ','')
    input=' '*c+input
    print(input)

# Driver program
if __name__ == "__main__":
    input = 'geeks for geeks'
    moveSpaces(input)

Output

  geeksforgeeks