## 题目描述：

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 `F` represent True and False respectively).

Note:

1. The length of the given string is ≤ 10000.
2. Each number will contain only one digit.
3. The conditional expressions group right-to-left (as usual in most languages).
4. The condition will always be either `T` or `F`. That is, the condition will never be a digit.
5. 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"
```

## 题目大意：

1. 字符串长度 ≤ 10000
2. 每个数字只有一位
3. 条件表达式从右向左分组 (与大多数编程语言保持一致)
4. 条件永远是 T 或者 F。 也就是说，条件永远不会是数字
5. 表达式的结果可以是 0-9, T 或者 F

## Python代码：

``````class Solution(object):
def parseTernary(self, expression):
"""
:type expression: str
:rtype: str
"""
stack = []
expr = list(expression)
while len(stack) > 1 or expr:
tail = stack[-5:]
if len(tail) == 5 and tail[3] == '?' and tail[1] == ':':
tail = tail[2] if tail[4] == 'T' else tail[0]
stack = stack[:-5] + [tail]
else:
stack.append(expr.pop())
return stack[0] if stack else None
``````

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