Problem Description

You are given an array of strings called words and a character called separator. Your task is to split each string in the words array using the separator character as a delimiter.

The problem asks you to:

1. Go through each string in the words array
2. Split each string wherever the separator character appears
3. Collect all the resulting substring pieces into a new array
4. Remove any empty strings from the result
5. Maintain the original order of the strings

Important details:

• The separator character itself should not be included in the resulting strings - it only marks where to split
• When a string is split, it might produce more than two parts (if the separator appears multiple times)
• Empty strings that result from the splitting process should be excluded from the final answer
• The order of strings in the result should follow the order they appeared in the original array

For example, if words = ["hello.world", "test..case"] and separator = ".", the splitting process would:

• Split "hello.world" into ["hello", "world"]
• Split "test..case" into ["test", "", "case"] (but the empty string would be removed)
• Return ["hello", "world", "test", "case"]

The solution uses a list comprehension that iterates through each word w in words, splits it by the separator, and only includes non-empty strings s in the final result.

Intuition

The problem is essentially asking us to perform a two-step process: split strings and then filter out empty results. When we think about this naturally, we would:

1. Take each word one by one
2. Split it based on the separator
3. Collect all the pieces
4. Remove any empty pieces

Python's built-in split() method already handles the splitting part perfectly - it breaks a string into parts wherever it finds the separator. The key insight is that we need to process multiple words and combine all their split results into a single flat list.

Instead of using nested loops explicitly, we can leverage Python's list comprehension to elegantly express this pattern. The nested comprehension [s for w in words for s in w.split(separator)] naturally flattens the results - it's like saying "for each word, split it, and for each piece from that split, add it to our result."

The filtering part comes from recognizing that when we split strings like "a..b" with separator ".", we get ['a', '', 'b'] - those empty strings need to be removed. Adding the condition if s at the end of the comprehension elegantly filters these out, since empty strings evaluate to False in Python.

This approach is intuitive because it mirrors exactly how we'd manually solve this problem: go through each word, break it apart at the separator, keep the non-empty pieces, and collect everything in order. The list comprehension just expresses this process concisely in a single line.

Solution Approach

The solution uses a simulation approach where we traverse through each string in the array and perform the split operation. Here's how the implementation works:

We iterate through the string array words and for each string w, we apply the split(separator) method. This built-in method divides the string into substrings wherever the separator character appears.

The implementation uses a nested list comprehension with three key components:

1. Outer iteration: for w in words - This iterates through each string in the input array
2. Inner iteration: for s in w.split(separator) - For each word w, we split it using the separator and iterate through the resulting substrings
3. Filtering condition: if s - This ensures we only include non-empty strings in our result

The complete list comprehension [s for w in words for s in w.split(separator) if s] effectively flattens and filters the results in a single pass.

Let's trace through an example:

• If words = ["one.two.three", "four..five"] and separator = "."
• For "one.two.three": split(".") gives ["one", "two", "three"]
• For "four..five": split(".") gives ["four", "", "five"]
• The if s condition filters out the empty string
• Final result: ["one", "two", "three", "four", "five"]

The time complexity is O(n × m) where n is the total number of characters across all strings and m is the average number of separators per string. The space complexity is O(n) for storing the result array.

This approach maintains the required order naturally since we process the words sequentially and append the split results in the order they appear.

Example Walkthrough

Let's walk through a concrete example to see how the solution works step by step.

Input: words = ["abc.def", "ghi..jkl", "m"], separator = "."

Step 1: Process the first word "abc.def"

• Apply split(".") → ["abc", "def"]
• Both strings are non-empty, so both are included
• Current result: ["abc", "def"]

Step 2: Process the second word "ghi..jkl"

• Apply split(".") → ["ghi", "", "jkl"]
• We get an empty string in the middle because there are two consecutive dots
• Filter with if s condition:
  • "ghi" is non-empty → include it
  • "" is empty → skip it
  • "jkl" is non-empty → include it
• Current result: ["abc", "def", "ghi", "jkl"]

Step 3: Process the third word "m"

• Apply split(".") → ["m"]
• No separator found, so the whole string becomes one element
• "m" is non-empty → include it
• Final result: ["abc", "def", "ghi", "jkl", "m"]

The list comprehension [s for w in words for s in w.split(separator) if s] handles all these steps in one expression:

• The outer loop (for w in words) goes through each word
• The inner loop (for s in w.split(separator)) processes each split piece
• The condition (if s) ensures only non-empty strings make it to the final result

This maintains the original order and correctly handles edge cases like consecutive separators or strings without any separators.

Solution Implementation

Time and Space Complexity

Time Complexity: O(n × m)

The algorithm iterates through each word in the words list (n words total). For each word, it performs a split() operation which takes O(m) time where m is the length of the current word (in worst case, the maximum length among all words). The split operation needs to scan through the entire string to find all occurrences of the separator. Therefore, the overall time complexity is O(n × m) where n is the number of words and m is the maximum length of strings in the array.

Space Complexity: O(m)

The space complexity is determined by the temporary space used during the split operation. When w.split(separator) is called, it creates a list of substrings from the word w. In the worst case, if a word of maximum length m contains no separators, the split operation creates a list containing the entire word, requiring O(m) space. Although the final result list might contain multiple split words, the space analysis focuses on the auxiliary space used during computation, not the output space. The list comprehension processes one word at a time, so the maximum auxiliary space needed at any point is O(m).

Learn more about how to find time and space complexity quickly.

Common Pitfalls

1. Confusing Split Behavior with Multiple Consecutive Separators

The Pitfall: Many developers incorrectly assume that consecutive separators will be treated as a single separator. In reality, each separator creates a split point, potentially generating empty strings between consecutive separators.

For example, with word = "a..b" and separator = ".":

• The split operation produces ["a", "", "b"] (not ["a", "b"])
• The empty string in the middle represents the "nothing" between the two dots

Why This Happens: The split() method treats each occurrence of the separator as a boundary. When two separators appear consecutively, there's technically an empty substring between them.

The Solution: The code correctly handles this by filtering out empty strings with the if part: condition. This ensures that only non-empty substrings make it to the final result.

2. Assuming Single Character Separator

The Pitfall: While the problem typically uses a single character as a separator, Python's split() method actually accepts strings of any length. If you accidentally pass a multi-character string as the separator, the behavior changes:

# Intended: separator = "."
# Accident: separator = ".."
word = "hello..world"
word.split("..")  # Returns ["hello", "world"]
word.split(".")   # Returns ["hello", "", "world"]

The Solution: Always verify the separator is exactly what you expect. If the problem guarantees a single character, you could add validation:

def splitWordsBySeparator(self, words: List[str], separator: str) -> List[str]:
    # Optional validation
    if len(separator) != 1:
        raise ValueError("Separator must be a single character")
  
    result = []
    for word in words:
        split_parts = word.split(separator)
        for part in split_parts:
            if part:
                result.append(part)
    return result

3. Edge Cases with Empty Input

The Pitfall: Not considering edge cases like:

• Empty words array: []
• Words containing only separators: ["...", "..."]
• Empty strings in the input: ["", "hello", ""]

Why This Matters: These edge cases can cause unexpected behavior or errors if not handled properly.

The Solution: The current implementation handles these gracefully:

• Empty array returns empty result
• Strings with only separators produce empty splits that get filtered out
• Empty strings in input produce no output (correctly filtered)

Test these edge cases explicitly:

# Edge case testing
assert splitWordsBySeparator([], ".") == []
assert splitWordsBySeparator(["..."], ".") == []
assert splitWordsBySeparator(["", "a.b", ""], ".") == ["a", "b"]