Is Ternary Expression Parser easy or hard?

Ternary Expression Parser is generally classified as a medium difficulty problem. The challenge comes from understanding that ternary expressions associate from right to left and designing a parsing strategy that correctly evaluates nested conditions.

Ternary Expression Parser Python/Java solution

Python, Java, C++, and Go implementations typically follow the same stack strategy. Iterate from the end of the string, push characters onto a stack, and when encountering a condition with '?', resolve the two candidate values and push the chosen result back.

How to solve Ternary Expression Parser in O(n)?

Traverse the expression from right to left and maintain a stack of characters. When encountering a condition ('T' or 'F') followed by '?', pop the true and false values from the stack and push back the selected result. This collapses each ternary segment exactly once, producing an O(n) solution.

What is the best approach for Ternary Expression Parser?

The stack-based right-to-left traversal is the most reliable approach. Scan the string from the end, push characters onto a stack, and resolve ternary segments whenever you see a condition followed by '?'. This evaluates nested expressions in one pass with O(n) time and O(n) space.

Is Ternary Expression Parser asked at Google/Amazon/Meta?

Ternary expression evaluation problems appear in interviews at large tech companies including Google, Amazon, and Meta because they test stack usage, parsing logic, and understanding of expression associativity. Variants also appear in compiler or interpreter style interview questions.

What data structure is used in Ternary Expression Parser?

A stack is the primary data structure used in the optimal solution. It helps evaluate nested ternary expressions while scanning the string from right to left. Recursion can also be used, which implicitly relies on the call stack.

What is the time complexity of Ternary Expression Parser?

The optimal solutions run in O(n) time where n is the length of the expression. Each character is processed once during the right-to-left scan and stack operations are constant time. Space complexity is O(n) due to the stack or recursion call stack.

Description Solution Video 11:26

439. Ternary Expression Parser

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Given a string expression representing arbitrarily nested ternary expressions, evaluate the expression, and return the result of it.

You can always assume that the given expression is valid and only contains digits, '?', ':', 'T', and 'F' where 'T' is true and 'F' is false. All the numbers in the expression are one-digit numbers (i.e., in the range [0, 9]).

The conditional expressions group right-to-left (as usual in most languages), and the result of the expression will always evaluate to either a digit, 'T' or 'F'.

Example 1:

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

Example 2:

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"
or "(F ? 1 : (T ? 4 : 5))" --> "(T ? 4 : 5)" --> "4"

Example 3:

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 : 3)" --> "F"
"(T ? (T ? F : 5) : 3)" --> "(T ? F : 5)" --> "F"

Constraints:

5 <= expression.length <= 10⁴
expression consists of digits, 'T', 'F', '?', and ':'.
It is guaranteed that expression is a valid ternary expression and that each number is a one-digit number.