Maximum Score From Removing Substrings

lcpy gen -n 1717   # by problem number
lcpy gen -s maximum_score_from_removing_substrings   # by problem name

Input: s = "cdbcbbaaabab", x = 4, y = 5
Output: 19
Explanation:
- Remove the "ba" underlined in "cdbcbbaaabab". Now, s = "cdbcbbaaab" and 5 points are added to the score.
- Remove the "ab" underlined in "cdbcbbaaab". Now, s = "cdbcbbaa" and 4 points are added to the score.
- Remove the "ba" underlined in "cdbcbbaa". Now, s = "cdbcba" and 5 points are added to the score.
- Remove the "ba" underlined in "cdbcba". Now, s = "cdbc" and 5 points are added to the score.
Total score = 5 + 4 + 5 + 5 = 19.

Input: s = "aabbaaxybbaabb", x = 5, y = 4
Output: 20

class Solution:
    # Time: O(n)
    # Space: O(n)
    def maximum_gain(self, s: str, x: int, y: int) -> int:
        hi_first, hi_second, hi = "a", "b", x
        lo_first, lo_second, lo = "b", "a", y
        if x < y:
            hi_first, hi_second, hi = "b", "a", y
            lo_first, lo_second, lo = "a", "b", x

        total = 0
        stack: list[str] = []
        for ch in s:
            if stack and stack[-1] == hi_first and ch == hi_second:
                stack.pop()
                total += hi
            else:
                stack.append(ch)

        leftover: list[str] = []
        for ch in stack:
            if leftover and leftover[-1] == lo_first and ch == lo_second:
                leftover.pop()
                total += lo
            else:
                leftover.append(ch)
        return total