1844. Replace All Digits with Characters

// Approach: For each even-index letter, shift the following digit to produce the target char.
// Time: O(n) Space: O(n)

public class Solution
{
    public string ReplaceDigits(string s)
    {
        char[] chars = s.ToCharArray();
        for (int i = 1; i < chars.Length; i += 2)
            chars[i] = (char)(chars[i] + chars[i - 1] - '0');

        return new string(chars);
    }
}