(Easy) Remove All Adjacent Duplicates In String LeetCode

class Solution {
    public String removeDuplicates(String S) {
        
      String result ="";
              int len = S.length();
            
            Stack <Character> st = new Stack<Character>();
            
            st.push(S.charAt(len-1));
            
        
        
        if(S == null || S.length() ==0 )
        {
               return "";
        }
         
        
        else if(S.length()==1){
            
            return S;
        }
        
        else {
            
    
            for(int i = S.length()-2; i>=0;i--){
                 if(st.empty()){
                  st.push(S.charAt(i));
                
              }
                else{
                    
                     if(S.charAt(i)== st.peek()){
                    st.pop();
                    
                }
                
                else{
                    st.push(S.charAt(i));
                }   
                    
                }
               
            
          }
        
           if(st.empty()){
               return result;
           } 
            else{
                
                do{
                    result = result + st.pop();
                    
                }
                while(!st.empty());
                
                return result;
            }
                
        }
 }
}