思路：
1. 用stack 如果比top的小以及后面还有剩余的话就可以pop和加上去 （在前面没有这个字母的情况下才考虑去加）


代码：

class Solution {
    public String smallestSubsequence(String text) {
        Map<Character,Integer>map=new HashMap<>();
        Map<Character,Integer>map1=new HashMap<>();
        Stack<Character>stack=new Stack<>();
        for(char c:text.toCharArray()){
            if(!map.containsKey(c))map.put(c,0);
            map.put(c,map.get(c)+1);
        }
        
        
        for(int i=0;i<text.length();i++){
            char c=text.charAt(i);
            map.put(c,map.get(c)-1);
            if(map.get(c)==0)map.remove(c);
            
            if(map1.containsKey(c))continue;
            
            while(stack.size()!=0&&map.containsKey(stack.peek())&&c<stack.peek()){
                char top=stack.pop();
                map1.put(top,map1.get(top)-1);
                if(map1.get(top)==0)map1.remove(top);
            }
            stack.push(c);
            if(!map1.containsKey(c))map1.put(c,0);
            map1.put(c,map1.get(c)+1);
        }
        
        
        
        StringBuilder str=new StringBuilder();
        while(stack.size()!=0){
            str.append(""+stack.pop());
        }
        
        return str.reverse().toString();
    }
}