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)
}