Solution.go

func evaluate(expression string) int {
	i, n := 0, len(expression)
	scope := map[string][]int{}

	parseVar := func() string {
		j := i
		for ; i < n && expression[i] != ' ' && expression[i] != ')'; i++ {
		}
		return expression[j:i]
	}

	parseInt := func() int {
		sign, v := 1, 0
		if expression[i] == '-' {
			sign = -1
			i++
		}
		for ; i < n && expression[i] >= '0' && expression[i] <= '9'; i++ {
			v = (v * 10) + int(expression[i]-'0')
		}
		return sign * v
	}

	var eval func() int
	eval = func() int {
		if expression[i] != '(' {
			if unicode.IsLower(rune(expression[i])) {
				t := scope[parseVar()]
				return t[len(t)-1]
			}
			return parseInt()
		}
		i++
		ans := 0
		if expression[i] == 'l' {
			i += 4
			vars := []string{}
			for {
				v := parseVar()
				if expression[i] == ')' {
					t := scope[v]
					ans = t[len(t)-1]
					break
				}
				i++
				vars = append(vars, v)
				scope[v] = append(scope[v], eval())
				i++
				if !unicode.IsLower(rune(expression[i])) {
					ans = eval()
					break
				}
			}
			for _, v := range vars {
				scope[v] = scope[v][:len(scope[v])-1]
			}
		} else {
			add := expression[i] == 'a'
			if add {
				i += 4
			} else {
				i += 5
			}
			a := eval()
			i++
			b := eval()
			if add {
				ans = a + b
			} else {
				ans = a * b
			}
		}
		i++
		return ans
	}
	return eval()
}