The Ternary Expression Parser coding problem involves evaluating a string that represents nested ternary expressions. A ternary expression follows the format condition ? expression1 : expression2. In this problem, the condition is always a single character ('T' or 'F'), and the expressions can be digits or further nested ternary expressions. Your task is to parse this string and return the result of the evaluation as a single character.
Snap and Snapchat frequently use this "Medium" level question to evaluate a candidate's understanding of expression parsing and stack-based algorithms. It tests your ability to handle nested structures and right-to-left evaluation. Solving the Ternary Expression Parser interview question requires you to demonstrate that you can manage state efficiently and handle the specific syntax of a custom expression language.
The primary algorithmic pattern is the Stack or Recursion pattern. Because ternary expressions are right-associative (nested from right to left), the most intuitive way to solve it is to parse the string from right to left.
Expression: F?1:T?4:5
T?4:5. Condition is T, so the result is 4. Push 4 to stack.F?1:4. Condition is F, so the result is 4.
Result: "4".
By evaluating the rightmost ternary first, we correctly resolve the nesting.A common mistake is trying to parse from left to right, which is much harder because you don't know where one expression ends and another begins without a more complex recursive descent parser. Another error is not correctly handling the stack pops for the true and false branches—remember that with right-to-left parsing, the "true" branch might have been pushed after the "false" branch depending on your implementation.
When preparing for the Ternary Expression Parser coding problem, practice parsing other nested structures like "Basic Calculator" or "Decode String." These all rely on the Stack interview pattern. Understanding the difference between left-to-right and right-to-left evaluation is a key skill for any parsing-related task.