algoadvance

You are given a 0-indexed string s that has lowercase English letters in its even indices and digits in its odd indices.

There is a function shift(c, x), where c is a character and x is a digit, that shifts c forward by x positions in the alphabet. For example, shift('a', 1) = 'b' and shift('x', 3) = 'a'.

Replace every digit in s with the character it shifts to and return the resulting string.

Clarifying Questions

1. What should be the output if the input string s is empty?
   • In this case, the output should be an empty string.
2. Is it guaranteed that the input string will always have digits at odd indices and letters at even indices?
   • Yes, the problem guarantees that the input format is valid.

Strategy

1. Convert the input string s to a list of characters to allow for mutable operations.
2. Iterate through the string with a step of 2, since every even index contains a letter and the subsequent odd index contains the corresponding digit.
3. Utilize the shift function to shift the character by the numeric value and replace the digit at the respective odd index with the shifted character.
4. Finally, convert the list back to a string and return the result.

Time Complexity

• The time complexity of this approach is O(n), where n is the length of the string s, since we are iterating through the string once and performing constant-time operations inside the loop.

Code

def shift(c, x):
    return chr(ord(c) + x)

def replaceDigits(s: str) -> str:
    chars = list(s)
    for i in range(1, len(chars), 2):
        chars[i] = shift(chars[i-1], int(chars[i]))
    return ''.join(chars)

# Example usage:
s = "a1c1e1"
print(replaceDigits(s))  # Output: "abcdef"

In this solution:

• The shift function calculates the new character by converting the character to its ASCII value using ord(c), adding the digit x, and converting back to a character using chr().
• The replaceDigits function iterates over the string, performing the necessary transformations and returns the modified string.

This approach ensures that we correctly replace all digits in the string with their corresponding shifted characters efficient.

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