Shortest Common Supersequence

class Solution:
    def shortestCommonSupersequence(self, str1: str, str2: str) -> str:
        memo = {}

        def solve(i, j):
            # Base Case: If one string is empty, return the rest of the other string
            if i == len(str1): return str2[j:]
            if j == len(str2): return str1[i:]

            # Check memoization table
            if (i, j) in memo:
                return memo[(i, j)]

            # If characters match, I take it once and move forward in both
            if str1[i] == str2[j]:
                res = str1[i] + solve(i + 1, j + 1)
            else:
                # Option 1: Take character from str1
                option1 = str1[i] + solve(i + 1, j)
                # Option 2: Take character from str2
                option2 = str2[j] + solve(i, j + 1)

                # Pick the shorter result
                if len(option1) < len(option2):
                    res = option1
                else:
                    res = option2

            memo[(i, j)] = res
            return res

        return solve(0, 0)