0150. Evaluate Reverse Polish Notation.py

# Evaluate the value of an arithmetic expression in Reverse Polish Notation.
# Valid operators are +, -, *, and /. Each operand may be an integer or another expression.
# Note that division between two integers should truncate toward zero.
# It is guaranteed that the given RPN expression is always valid. That means the expression would always evaluate to a result, and there will not be any division by zero operation.
#
# Example 1:
#
# Input: tokens = ["2","1","+","3","*"]
# Output: 9
# Explanation: ((2 + 1) * 3) = 9
#
# Example 2:
#
# Input: tokens = ["4","13","5","/","+"]
# Output: 6
# Explanation: (4 + (13 / 5)) = 6
#
# Example 3:
#
# Input: tokens = ["10","6","9","3","+","-11","*","/","*","17","+","5","+"]
# Output: 22
# Explanation: ((10 * (6 / ((9 + 3) * -11))) + 17) + 5
# = ((10 * (6 / (12 * -11))) + 17) + 5
# = ((10 * (6 / -132)) + 17) + 5
# = ((10 * 0) + 17) + 5
# = (0 + 17) + 5
# = 17 + 5
# = 22
#
# Constraints:
#
# 1 <= tokens.length <= 104
# tokens[i] is either an operator: "+", "-", "*", or "/", or an integer in the range [-200, 200].


class Solution:
    def evalRPN(self, tokens: List[str]) -> int:
        st = []
        for i in tokens:
            if i in ["+", "-", "*", "/"]:
                a = st.pop()
                b = st.pop()
                if i == "+":
                    st.append(b + a)
                elif i == "-":
                    st.append(b - a)
                elif i == "*":
                    st.append(b * a)
                else:
                    st.append(int(b / a))
            else:
                st.append(int(i))
        return st[0]