224. Basic Calculator

Implement a basic calculator to evaluate a simple expression string.

The expression string may contain open ( and closing parentheses ), the plus + or minus sign -, non-negative integers and empty spaces .

Example 1:

Input: "1 + 1"
Output: 2

Example 2:

Input: " 2-1 + 2 "
Output: 3

Example 3:

Input: "(1+(4+5+2)-3)+(6+8)"
Output: 23

Note:

• You may assume that the given expression is always valid.
• Do not use the eval built-in library function.

# @lc code=start
using LeetCode

function calculate(s::String)::Int
    res, num, sign, stk = 0, 0, 1, []
    for c in s
        if isdigit(c)
            num = 10 * num + parse(Int, c)
        elseif c in ['-', '+']
            res += num * sign
            num = 0
            sign = (c == '-') ? -1 : 1
        elseif c == '('
            push!(stk, res)
            push!(stk, sign)
            res, sign = 0, 1
        elseif c == ')'
            res += num * sign
            res *= pop!(stk)
            res += pop!(stk)
            num = 0
        end
    end

    return res + num * sign
end
# @lc code=end

calculate (generic function with 1 method)

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