func addStrings(num1 string, num2 string) string {
	m, n := len(num1), len(num2)
	n1, n2 := []rune(num1), []rune(num2)
	if m < n {
		m, n = n, m
		n1, n2 = n2, n1
	}
	carry := '0'
	for i, j := m-1, n-1; 0 <= i; i, j = i-1, j-1 {
		var b rune
		if j < 0 {
			if carry == '0' {
				break
			}
			b = '0'
		} else {
			b = n2[j]
		}
		n1[i], carry = add(n1[i], b, carry)
	}
	if carry != '0' {
		return "1" + string(n1)
	}
	return string(n1)
}

func add(a, b, c rune) (rune, rune) {
	num := a + b + c - 144 // 3 * '0'
	return num%10 + '0', num/10 + '0'
}