[LeetCode 439] Ternary Expression Parser

Given a string representing arbitrarily nested ternary expressions, calculate the result of the expression. You can always assume that the given expression is valid and only consists of digits 0-9, ?, :, T and F (T and Frepresent True and False respectively).

Note:

• 1. The length of the given string is ≤ 10000.
• 1. Each number will contain only one digit.
• 1. The conditional expressions group right-to-left (as usual in most languages).
• 1. The condition will always be either T or F. That is, the condition will never be a digit.
• 1. The result of the expression will always evaluate to either a digit 0-9, T or F.

Example 1:

Input: "T?2:3"

Output: "2"

Explanation: If true, then result is 2; otherwise result is 3.

Example 2:

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

Example 3:

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

Diffculty
Medium

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