package main

// just like "add two numbers"
func plusOne(digits []int) []int {
	i, remain := len(digits)-1, 0
	for digits[len(digits)-1] = digits[len(digits)-1] + 1; i >= 0; i-- {
		digits[i] += remain
		remain = int(digits[i] / 10)
		digits[i] %= 10
	}
	// way to concat two slice: append(a, b...)
	if remain == 1 {
		return append([]int{1}, digits...)
	}
	return digits
}