1106. Parsing A Boolean Expression

Return the result of evaluating a given boolean expression, represented as a string.

An expression can either be:

• "t", evaluating to True;
• "f", evaluating to False;
• "!(expr)", evaluating to the logical NOT of the inner expression expr;
• "&(expr1,expr2,...)", evaluating to the logical AND of 2 or more inner expressions expr1, expr2, ...;
• "|(expr1,expr2,...)", evaluating to the logical OR of 2 or more inner expressions expr1, expr2, ...

 

Example 1:

Input: expression = "!(f)"
Output: true

Example 2:

Input: expression = "|(f,t)"
Output: true

Example 3:

Input: expression = "&(t,f)"
Output: false

Example 4:

Input: expression = "|(&(t,f,t),!(t))"
Output: false

 

Constraints:

• 1 <= expression.length <= 20000
• expression[i] consists of characters in {'(', ')', '&', '|', '!', 't', 'f', ','}.
• expression is a valid expression representing a boolean, as given in the description.

思路：栈，还是逆序波兰表达，更easy

class Solution(object):
    def parseBoolExpr(self, expression):
        """
        :type expression: str
        :rtype: bool
        """
        st=[]
        for s in expression:
            if s==')':
                A=[]
                while st:
                    a=st.pop()
                    if a=='(': break
                    A.append(a)
                op=st.pop()
                if op=='&': st.append('f' if 'f' in A else 't')
                elif op=='|': st.append('t' if 't' in A else 'f')
                else: st.append('t' if A[0]=='f' else 'f')
            else:
                st.append(s)
                
        return True if st[-1]=='t' else False 
    
s=Solution()
print(s.parseBoolExpr("!(f)"))
print(s.parseBoolExpr("|(f,t)"))
print(s.parseBoolExpr("&(t,f)"))
print(s.parseBoolExpr("|(&(t,f,t),!(t))"))