439. Ternary Expression Parser

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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 F represent 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 either T or F. That is, the condition will never be a digit.
The result of the expression will always evaluate to either a digit 0-9, T or F.

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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"

解题思路:

先对字符串反转,每次向后读取进栈,如果遇到问号,则根据问号后是T还是F来弹栈,如果是T,则将最后一位保留,倒数第二位:和第三位删除,否则删除倒数第一和第二位,其他的保留。

python

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class Solution(object):
def parseTernary(self, expression):
"""
:type expression: str
:rtype: str
"""
expression = expression[::-1]
res = []
i = 0
while i < len(expression):
if expression[i] != '?':
res.append(expression[i])
else:
i += 1
if expression[i] == 'F':
res = res[:-3] + [res[-3]]
elif expression[i] == 'T':
res = res[:-3] + [res[-1]]
i += 1
return res[-1]
谢谢你,可爱的朋友。