func removeDuplicateLetters(s string) string {
	cnt := make([]int, 256)
	for _, c := range s {
		cnt[c]++
	}
	set := make([]bool, 256)
	res := make([]rune, 0)
	tp := -1
	for _, c := range s {
		cnt[c]--
		if set[c] {
			continue
		}
		// If c is not in stack && stack is not empty && c less than top && top'll appear again, pop top
		for tp != -1 && c < res[tp] && 0 < cnt[res[tp]] {
			set[res[tp]] = false
			res = res[:tp]
			tp--
		}
		res = append(res, c)
		set[c] = true
		tp++
	}
	return string(res)
}