Problem Description

You are given a string s. Your task is to calculate the score of this string.

The score is calculated by:

1. Looking at each pair of adjacent characters in the string
2. Finding the ASCII value of each character
3. Computing the absolute difference between these ASCII values
4. Summing up all these absolute differences

For example, if you have a string "abc":

• The ASCII value of 'a' is 97, 'b' is 98, and 'c' is 99
• The absolute difference between 'a' and 'b' is |97 - 98| = 1
• The absolute difference between 'b' and 'c' is |98 - 99| = 1
• The total score would be 1 + 1 = 2

The solution uses Python's pairwise function to iterate through consecutive pairs of characters, ord to get ASCII values, and calculates the sum of absolute differences between each pair.

Intuition

The problem asks us to find the sum of absolute differences between adjacent characters' ASCII values. This is a straightforward simulation problem where we need to process the string character by character.

The key insight is that we need to:

1. Process each adjacent pair of characters exactly once
2. Convert each character to its ASCII value
3. Calculate the absolute difference
4. Sum all these differences

Since we're dealing with adjacent pairs, if we have a string of length n, we'll have n-1 pairs to process. For example, in "hello" we have pairs: (h,e), (e,l), (l,l), (l,o).

The natural approach is to iterate through the string and for each position i from 0 to n-2, calculate |ord(s[i]) - ord(s[i+1])| and accumulate the sum.

Python's pairwise function elegantly handles the pairing of adjacent elements, eliminating the need for manual index management. By combining it with map(ord, s) to convert all characters to ASCII values first, we can then simply calculate the absolute difference for each pair and sum them up in a single expression.

This leads to a clean one-liner solution: iterate through pairs of ASCII values and sum their absolute differences.

Solution Approach

The solution follows a simulation approach where we directly compute what the problem asks for - the sum of absolute differences between adjacent characters' ASCII values.

Implementation Details:

1. Convert characters to ASCII values: We use map(ord, s) to transform each character in the string to its corresponding ASCII value. The ord() function returns the Unicode/ASCII code point of a character.

2. Create adjacent pairs: The pairwise() function from Python's itertools (available in Python 3.10+) takes an iterable and returns successive overlapping pairs. For example, if we have ASCII values [97, 98, 99] for "abc", pairwise would give us pairs (97, 98) and (98, 99).

3. Calculate absolute differences: For each pair (a, b) returned by pairwise, we compute abs(a - b) to get the absolute difference between the ASCII values.

4. Sum all differences: The sum() function aggregates all these absolute differences to produce the final score.

The entire solution is expressed as a single line using a generator expression:

sum(abs(a - b) for a, b in pairwise(map(ord, s)))

This approach has:

• Time Complexity: O(n) where n is the length of the string, as we traverse the string once
• Space Complexity: O(1) as we only use a constant amount of extra space (the generator expression doesn't create intermediate lists)

Example Walkthrough

Let's walk through the solution with the string s = "hello".

Step 1: Convert characters to ASCII values

• 'h' → 104
• 'e' → 101
• 'l' → 108
• 'l' → 108
• 'o' → 111

So we have the sequence: [104, 101, 108, 108, 111]

Step 2: Create adjacent pairs using pairwise The pairwise function gives us:

• Pair 1: (104, 101) for ('h', 'e')
• Pair 2: (101, 108) for ('e', 'l')
• Pair 3: (108, 108) for ('l', 'l')
• Pair 4: (108, 111) for ('l', 'o')

Step 3: Calculate absolute differences for each pair

• Pair 1: |104 - 101| = 3
• Pair 2: |101 - 108| = 7
• Pair 3: |108 - 108| = 0
• Pair 4: |108 - 111| = 3

Step 4: Sum all differences Total score = 3 + 7 + 0 + 3 = 13

The one-liner solution sum(abs(a - b) for a, b in pairwise(map(ord, s))) performs all these steps:

• map(ord, s) converts "hello" to ASCII values
• pairwise(...) creates the 4 adjacent pairs
• abs(a - b) for a, b in ... calculates each absolute difference
• sum(...) adds them all up to get 13

Solution Implementation

Time and Space Complexity

The time complexity is O(n), where n is the length of the string s. This is because:

• map(ord, s) iterates through all characters in the string once: O(n)
• pairwise() creates pairs of consecutive elements, requiring a single pass: O(n-1)
• sum() iterates through all pairs to calculate the sum: O(n-1)
• The abs(a - b) operation for each pair is O(1)

Overall, the algorithm makes a linear pass through the string, resulting in O(n) time complexity.

The space complexity is O(1). Although pairwise() and map() create iterators, they don't store all elements in memory simultaneously. The iterators process elements one at a time, using only constant extra space for:

• The current pair of characters being processed
• The running sum accumulator
• Temporary variables for the absolute difference calculation

Therefore, the space usage remains constant regardless of the input string length.

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

Common Pitfalls

1. Python Version Compatibility Issue

The pairwise function is only available in Python 3.10+. If you're using an older Python version or submitting to a platform that doesn't support Python 3.10+, the code will fail with an ImportError.

Solution: Implement your own pairwise logic using zip:

class Solution:
    def scoreOfString(self, s: str) -> int:
        # Works in all Python versions
        return sum(abs(ord(s[i]) - ord(s[i+1])) for i in range(len(s) - 1))

Or using zip with slicing:

class Solution:
    def scoreOfString(self, s: str) -> int:
        # Create pairs using zip
        return sum(abs(ord(a) - ord(b)) for a, b in zip(s, s[1:]))

2. Edge Case: Empty or Single Character String

While the current solution handles single character strings correctly (returns 0), you might forget to consider these edge cases when implementing alternative solutions.

Solution: Always validate input:

class Solution:
    def scoreOfString(self, s: str) -> int:
        if len(s) <= 1:
            return 0
        return sum(abs(ord(s[i]) - ord(s[i+1])) for i in range(len(s) - 1))

3. Misunderstanding ASCII vs Unicode

Some developers might assume all characters will be standard ASCII (0-127), but Python's ord() returns Unicode code points which can be much larger for non-ASCII characters.

Solution: The current approach using abs() handles this correctly regardless of the character range, but be aware that the problem constraints should specify the character set if this matters.

4. Off-by-One Errors in Manual Iteration

When implementing without pairwise, a common mistake is iterating through all indices including the last one, causing an IndexError.

Wrong approach:

# This will cause IndexError
for i in range(len(s)):  # Should be range(len(s) - 1)
    diff = abs(ord(s[i]) - ord(s[i+1]))

Correct approach:

for i in range(len(s) - 1):  # Stop before the last character
    diff = abs(ord(s[i]) - ord(s[i+1]))