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2019.4.16

private class preorderIerator implements Iterator
   
    {
           private ArrayList
    
      list = new ArrayList<>();        private int current = 0;        public preorderIerator(){
               preorder();        }        private void preorder(){
               preorder(root);        }        private void preorder(TreeNode
     
       root){
               if(root == null)                return;            list.add(root.element);            preorder(root.left);            preorder(root.right);        }        @Override        public boolean hasNext() {
               return current < list.size();        }        @Override        public T next() {
               return list.get(current++);        }    }
     
    
   

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