150. Evaluate Reverse Polish Notation

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are +, -, *, /. Each operand may be an integer or another expression.

Note:

• Division between two integers should truncate toward zero.
• The given RPN expression is always valid. That means the expression would always evaluate to a result and there won't be any divide by zero operation.

Example 1:

Input: ["2", "1", "+", "3", "*"]
Output: 9
Explanation: ((2 + 1) * 3) = 9

Example 2:

Input: ["4", "13", "5", "/", "+"]
Output: 6
Explanation: (4 + (13 / 5)) = 6

Example 3:

Input: ["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

# @lc code=start
using LeetCode
function eval_rpn(tokens::Vector{String})
    stk = Stack{Int}()
    for token in tokens
        if token == "+"
            num2 = pop!(stk)
            num1 = pop!(stk)
            push!(stk, num1 + num2)
        elseif token == "-"
            num2 = pop!(stk)
            num1 = pop!(stk)
            push!(stk, num1 - num2)
        elseif token == "*"
            num2 = pop!(stk)
            num1 = pop!(stk)
            push!(stk, num1 * num2)
        elseif token == "/"
            num2 = pop!(stk)
            num1 = pop!(stk)
            push!(stk, num1 ÷ num2)
        else
            push!(stk, parse(Int, token))
        end
    end
    first(stk)
end

# @lc code=end

eval_rpn (generic function with 1 method)

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