Replace Question Symbols to Avoid Consecutive Repeating Characters in Python

Suppose we have a lowercase string s that contains only letters and '?' character, we have to convert all '?' characters into lower case letters such that the final string will not have any consecutive repeating characters. If there is more than one solution, return any of them.

So, if the input is like s = "hel??", then the output will be helab, se first question mark may be anything except 'l' and when first one is given, then second one can be anything except 'a'.

To solve this, we will follow these steps −

• if size of s is same as 1, then

  • if s is same as "?", then

    • return "a"

  • return s

• s := list of characters present in s

• for i in range 0 to size of s - 1, do

  • if s[i] is same as "?", then

    • if i is same as 0 and s[i+1] is same as "?", then

      • s[i] := "a"

    • otherwise when i is same as 0 and s[i+1] is same as "a", then

      • s[i] := "b"

    • otherwise when i is same as 0, then

      • s[i] := "a"

    • otherwise when i is same as (size of s)-1 and s[i-1] is same as "a", then

      • s[i] := "b"

    • otherwise when i is same as (size of s)-1, then

      • s[i] := "a"

    • otherwise when s[i-1] is same as "a" and s[i+1] is same as "?", then

      • s[i] := "b"

    • otherwise when s[i+1] is same as "?", then

      • s[i] := "a"

    • otherwise when (s[i-1] is same as "a" and s[i+1] is same as "b") or (s[i-1] is same as "b" and s[i+1] is same as "a"), then

      • s[i] := "c"

    • otherwise when s[i-1] or s[i+1] is "a", then

      • s[i] := "b"

    • otherwise,

      • s[i] := "a"

• return s after joining characters into string

Example (Python)

Let us see the following implementation to get better understanding −

 Live Demo

def solve(s):
   if len(s) == 1 :
      if s == "?":
         return "a"
      return s
   s = list(s)
   for i in range(len(s)):
      if s[i] == "?":
         if i == 0 and s[i+1] == "?":
            s[i] = "a"
         elif i == 0 and s[i+1] == "a":
            s[i] = "b"
         elif i == 0:
            s[i] = "a"
         elif i == len(s)-1 and s[i-1] == "a":
            s[i] = "b"
         elif i == len(s)-1:
            s[i] = "a"
         elif s[i-1] == "a" and s[i+1] == "?":
            s[i] = "b"
         elif s[i+1] == "?":
            s[i] = "a"
         elif (s[i-1] == "a" and s[i+1] == "b") or (s[i-1] == "b" and s[i+1] == "a"):
            s[i] = "c"
         elif "a" in (s[i-1],s[i+1]):
            s[i] = "b"
         else:
            s[i] = "a"
   return "".join(s)

s = "hel??"
print(solve(s))

Input

"hel??"

Output

helab