LeetCode 439. Ternary Expression Parser

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 digits0-9,?,:,TandF(TandFrepresent True and False respectively).

Note:

The length of the given string is ≤ 10000.

Each number will contain only one digit.

The conditional expressions group right-to-left (as usual in most languages).

The condition will always be eitherTorF. That is, the condition will never be a digit.

The result of the expression will always evaluate to either a digit0-9,TorF.

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"

思路：

1.用stack来做，从右往左遍历：遇到？，则根据？前的T/F从stack顶上两个数选一个保留；遇到数字或者F/T,直接push；遇到：不用操作！