LeetCode in Kotlin

224. Basic Calculator

Hard

Given a string s representing a valid expression, implement a basic calculator to evaluate it, and return the result of the evaluation.

Note: You are not allowed to use any built-in function which evaluates strings as mathematical expressions, such as eval().

Example 1:

Input: s = “1 + 1”

Output: 2

Example 2:

Input: s = “ 2-1 + 2 “

Output: 3

Example 3:

Input: s = “(1+(4+5+2)-3)+(6+8)”

Output: 23

Constraints:

• 1 <= s.length <= 3 * 105
• s consists of digits, '+', '-', '(', ')', and ' '.
• s represents a valid expression.
• '+' is not used as a unary operation (i.e., "+1" and "+(2 + 3)" is invalid).
• '-' could be used as a unary operation (i.e., "-1" and "-(2 + 3)" is valid).
• There will be no two consecutive operators in the input.
• Every number and running calculation will fit in a signed 32-bit integer.

Solution

class Solution {
    private var i = 0
    fun calculate(s: String): Int {
        val ca = s.toCharArray()
        return helper(ca)
    }

    private fun helper(ca: CharArray): Int {
        var num = 0
        var prenum = 0
        var isPlus = true
        while (i < ca.size) {
            val c = ca[i]
            if (c != ' ') {
                if (c >= '0' && c <= '9') {
                    num = if (num == 0) {
                        c.code - '0'.code
                    } else {
                        num * 10 + c.code - '0'.code
                    }
                } else if (c == '+') {
                    prenum += num * if (isPlus) 1 else -1
                    isPlus = true
                    num = 0
                } else if (c == '-') {
                    prenum += num * if (isPlus) 1 else -1
                    num = 0
                    isPlus = false
                } else if (c == '(') {
                    i++
                    prenum += helper(ca) * if (isPlus) 1 else -1
                    isPlus = true
                    num = 0
                } else if (c == ')') {
                    return prenum + num * if (isPlus) 1 else -1
                }
            }
            i++
        }
        return prenum + num * if (isPlus) 1 else -1
    }
}

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