Ternary Expression Parser Python Solution

lcpy gen -n 439   # by problem number
lcpy gen -s ternary_expression_parser   # by problem name

Input: expression = "T?2:3"
Output: "2"
Explanation: If true, then result is 2; otherwise result is 3.

Input: expression = "F?1:T?4:5"
Output: "4"
Explanation: The conditional expressions group right-to-left. Using parenthesis, it is read/evaluated as: "(F ? 1 : (T ? 4 : 5))" --> "(F ? 1 : 4)" --> "4".

Input: expression = "T?T?F:5:3"
Output: "F"
Explanation: The conditional expressions group right-to-left. Using parenthesis, it is read/evaluated as: "(T ? (T ? F : 5) : 3)" --> "(T ? F : 5)" --> "F".

class Solution:
    # Time: O(n)
    # Space: O(n)
    def parse_ternary(self, expression: str) -> str:
        stack: list[str] = []
        for ch in reversed(expression):
            if stack and stack[-1] == "?":
                stack.pop()
                true_val = stack.pop()
                stack.pop()  # ':'
                false_val = stack.pop()
                stack.append(true_val if ch == "T" else false_val)
            else:
                stack.append(ch)
        return stack[0]