Replace All Digits with Characters

This problem can be solved by iterating through the string s and performing the shift operation for the digits at the odd indices. Remember that Python strings are immutable, so we cannot directly change a character at a certain index. We will create a new result string to hold the modified string.

Python code:

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class Solution:
    def replaceDigits(self, s: str) -> str:
        result = ""
        for i in range(len(s)):
            # If the index is odd, perform the shift operation
            if i % 2 != 0:
                # ord() gives the Unicode code point of a character.
                # 'a' has a code point of 97, so to shift by s[i],
                # we subtract 97 from the code point of s[i-1],
                # add int(s[i]), and add 97 back.
                # chr() converts the resulting code point back to a character.
                result += chr(ord(s[i-1]) + int(s[i]))
            else:
                # If the index is even, just add the character to the result
                result += s[i]
        return result

In the replaceDigits method, we first initialize an empty string result. We then iterate over the length of the string s using a for loop.

For each index i, we check if i is odd by checking i % 2 != 0. If i is odd, we perform the shift operation on s[i-1] by int(s[i]) using the ord and chr functions and add the result to result. If i is even, we simply add s[i] to result.

Finally, we return the modified string result.