题目：

题解：

func removeDuplicateLetters(s string) string {left := [26]int{}for _, ch := range s {left[ch-'a']++}stack := []byte{}inStack := [26]bool{}for i := range s {ch := s[i]if !inStack[ch-'a'] {for len(stack) > 0 && ch < stack[len(stack)-1] {last := stack[len(stack)-1] - 'a'if left[last] == 0 {break}stack = stack[:len(stack)-1]inStack[last] = false}stack = append(stack, ch)inStack[ch-'a'] = true}left[ch-'a']--}return string(stack)
}