LeetCode Solutions Blog

Thousand Separator

Number: 1660

Difficulty: Easy

Paid? No

Companies: N/A

Problem Description

Given a non-negative integer n, convert it to a string and insert a dot “.” as the thousands separator. For example, 1234 becomes "1.234" and 987 remains "987".

Key Insights

Convert the integer to its string representation.
Process the string from right to left to group digits in sets of three.
Insert a dot between these groups.
Reverse the result to restore the original digit order.

Space and Time Complexity

Time Complexity: O(n) where n is the number of digits
Space Complexity: O(n) for storing the new string with separators

Solution

We first convert the given number to a string. Then, starting from the end of the string, we group the digits in batches of three. For each complete group (except possibly the first), a dot is added as a separator. Finally, we reverse the built string to obtain the thousands-separated number in correct order. This approach efficiently handles the grouping without converting back and forth from numeric types.

Code Solutions

# Define the function to format the number with a thousands separator
def thousandSeparator(n):
    # Convert the integer to string for easier manipulation
    s = str(n)
    # Initialize an empty list to build the result in reverse order
    parts = []
    # Counter for digits processed in the current group
    count = 0
    # Iterate over the string in reverse order
    for digit in s[::-1]:
        parts.append(digit)  # Append the current digit
        count += 1
        # If three digits have been processed and there are still digits remaining, add a dot separator
        if count == 3 and len(parts) < len(s) + (len(s) - 1) // 3:
            parts.append('.')
            count = 0  # Reset count after inserting separator
    # Reverse the list to restore the original order and join the list into a string
    return ''.join(parts[::-1])

# Example usage:
print(thousandSeparator(987))   # Output: "987"
print(thousandSeparator(1234))  # Output: "1.234"