1844 Replace All Digits with Characters

Intuition

Pretty straightforward, have a mutable form of the string like a list (or create new string and append to it). Then when you encounter a digit, calculate character for that index using the previous character and current digit.

Hint: for Python devs, use ord() to get ASCII value of a character and chr() to convert a digit into a character.

Code

Python3

def replaceDigits(self, s: str) -> str:
    v = list(s)
 
    for i in range(1, len(s)):
        if s[i].isdigit():
            v[i] = chr(ord(s[i-1]) + int(s[i]))
            continue
        
    return ''.join(v)

Big O Analysis

• Runtime

  The runtime complexity here is $O(N)$ since we would be visiting all characters in the string of length N.

• Memory

  The memory usage is $O(V)$ since we convert the input string into a list v.

— A

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