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！
