EvaluateExpression.kt

import java.util.Stack

fun evaluateExpression(s: String): Int {
    val stack = Stack<Int>()
    var currNum = 0
    var sign = 1
    var res = 0
    for (c in s) {
        if (c.isDigit()) {
            currNum = currNum * 10 + c.digitToInt()
            // If the current character is an operator, add 'curr_num' to
            // the result after multiplying it by its sign.
        } else if (c == '+' || c == '-') {
            res += currNum * sign
            // Update the sign and reset 'curr_num'.
            sign = if (c == '-') -1 else 1
            currNum = 0
            // If the current character is an opening parenthesis, a new
            // nested expression is starting.
        } else if (c == '(') {
            // Save the current 'res' and 'sign' values by pushing them
            // onto the stack, then reset their values to start
            // calculating the new nested expression.
            stack.push(res)
            stack.push(sign)
            res = 0
            sign = 1
            // If the current character is a closing parenthesis, a nested
            // expression has ended.
        } else if (c == ')') {
            // Finalize the result of the current nested expression.
            res += sign * currNum
            // Apply the sign of the current nested  expression's result
            // before adding this result to the result of the outer
            // expression.
            res *= stack.pop()
            res += stack.pop()
            currNum = 0
        }
    }
    // Finalize the result of the overall expression.
    return res + currNum * sign
}