439 Ternary Expression Parser
439. Ternary Expression Parser
题目: https://leetcode.com/problems/ternary-expression-parser/
难度: Medium
思路：
其实这个和算术运算蛮像，但是不同于运算，有operator precedence差别，这个是三目运算，并且需要检查是否符合运算规则。
运用stack 然后每次查看是否形成运算式再来做处理
AC代码：
class Solution(object):
def parseTernary(self, expression):
"""
:type expression: str
:rtype: str
"""
n = len(expression)
stack = []
for i in range(n-1, -1, -1):
char = expression[i]
stack.append(char)
if len(stack) >= 5:
op0 = stack.pop()
op1 = stack.pop()
op2 = stack.pop()
op3 = stack.pop()
op4 = stack.pop()
if op1 == '?' and op3 == ':':
res = op2 if op0 == 'T' else op4
stack.append(res)
else:
stack.append(op4)
stack.append(op3)
stack.append(op2)
stack.append(op1)
stack.append(op0)
return stack[0]
