# Problem Description
This is a string manipulation problem where we need to transform an input string by replacing characters at odd-indexed positions with a specific pattern. The goal is to replace each character at even indices (0-based indexing) with the string "qwe" while keeping odd-indexed characters unchanged.

# Visual Facts
1. Four examples are shown in the image:
   - Example 1: s = "a" → result = "qwe"
   - Example 2: s = "cat" → result = "qweaqwe"
   - Example 3: s = "world" → result = "qweoqwelaqwe"
   - Example 4: s = "freedom" → result = "qwerqweeqweoqwe"

2. Input strings have varying lengths:
   - Single character ("a")
   - Three characters ("cat")
   - Five characters ("world")
   - Seven characters ("freedom")

3. Each example shows the input string and its corresponding transformed result

# Visual Patterns
1. Pattern for character replacement:
   - Every character at an even-indexed position (0-based) is replaced with "qwe"
   - Characters at odd-indexed positions remain unchanged in their original positions

2. Transformation rules:
   - Single character input ("a") → "qwe"
   - For longer strings:
     * Odd-indexed characters (0-based) stay in place
     * Even-indexed characters (0-based) are replaced with "qwe"


# Solution
def solution(s: str) -> str:
    """
    Modifies the input string according to a specified pattern.

    Parameters:
        s (str): The input string to modify.

    Returns:
        str: The modified string.
    """
    result = []
    target = 'qwe'
    for i in range(len(s)):
        if i % 2 == 0:  # Check if the index is even
            result.append(target)  # Replace the letter with target
        else:
            result.append(s[i])  # Keep the original letter
    return ''.join(result)


# Test Script
def test():
    test_cases = [
        ('a', 'qwe'),
        ('abc', 'qwebqwe'),
        ('hello', 'qweeqwelqwe'),
        ('world', 'qweoqwelqwe'),
        ('cat', 'qweaqwe'),
        ('string', 'qwetqweiqweg'),
        ('check', 'qwehqwecqwe'),
        ('ocuihoduhfoiaugfoiusdviusbdics', 'qwecqweiqweoqweuqwefqweiqweuqwefqweiqwesqwevqweuqwebqweiqwes'),
        ('csudgcisug1231387948rf9ewufha', 'qwesqwedqwecqwesqwegqwe2qwe1qwe8qwe9qwe8qwefqweeqweuqwehqwe'),
        ('adiuevi0248ut9huodnfoir<><><;;eoifh1-=-s9uh`', 'qwedqweuqwevqwe0qwe4qweuqwe9qweuqwedqwefqweiqwe<qwe<qwe<qwe;qweoqwefqwe1qwe=qwesqweuqwe`')
    ]

    for i, (s, expected_output) in enumerate(test_cases):
        try:
            output = solution(s)
        except Exception as e:
            error_message = (
                f"An exception occurred while running the test case:\n"
                f"`s`: {s}\n\n"
                f"Exception: '{str(e)}'"
            )
            assert False, error_message

        if output != expected_output:
            error_message = (
                f"The following Test Case failed:\n"
                f"`s`: {s}\n\n"
                f"Actual Output: {output}\n"
                f"Expected Output: {expected_output}"
            )
            assert False, error_message

test()