3.go

func appendInt(s []int, val int) []int {
	idx := len(s)
	if len(s) < cap(s) {
        // Enough space, use same underlying array
		s = s[:len(s)+1]
	} else {
        // Not enough space, allocate & copy
		s1 := make([]int, 2*len(s)+1)
		copy(s1, s)
		s = s1[:len(s)+1]
	}
	s[idx] = val
	return s
}