import (
	"strconv"
	"strings"
)

func calculate(s string) int {
	none := struct{}{}
	opsSet := map[byte]struct{}{'+': none, '-': none, '*': none, '/': none}
	ops := make([]byte, 0)
	nums := make([]int, 0)
	prev := -1
	for i := range s {
		if _, ok := opsSet[s[i]]; ok {
			num, _ := strconv.Atoi(strings.Trim(s[prev+1:i], " ")) // Atoi should trim all spaces of string
			mulOrDiv(&nums, &ops, &num)
			nums = append(nums, num)
			ops = append(ops, s[i])
			prev = i
		}
	}
	num, _ := strconv.Atoi(strings.Trim(s[prev+1:], " "))
	mulOrDiv(&nums, &ops, &num)
	nums = append(nums, num)
	for left, right := 0, len(nums)-1; left < right; left, right = left+1, right-1 {
		nums[left], nums[right] = nums[right], nums[left]
	} // Reverse nums and ops
	for left, right := 0, len(ops)-1; left < right; left, right = left+1, right-1 {
		ops[left], ops[right] = ops[right], ops[left]
	}
	for l1, l2 := len(nums), len(ops); l1 != 1; l1, l2 = l1-1, l2-1 {
		op := ops[l2-1]
		ops = ops[:l2-1]
		n1, n2 := nums[l1-2], nums[l1-1]
		nums = nums[:l1-2]
		switch op {
		case '+':
			nums = append(nums, n2+n1)
		case '-':
			nums = append(nums, n2-n1)
		}
	}
	return nums[0]
}

func mulOrDiv(nums *[]int, ops *[]byte, num *int) {
	if l := len(*ops); l != 0 && ((*ops)[l-1] == '*' || (*ops)[l-1] == '/') {
		n := (*nums)[len(*nums)-1]
		*nums = (*nums)[:len(*nums)-1]
		switch (*ops)[l-1] {
		case '*':
			*num = n * (*num)
		case '/':
			*num = n / (*num)
		}
		*ops = (*ops)[:l-1]
	}
}