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哈希表法 先中序遍历搜索树 然后将值放进数组，然后一个一个取出来

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode() {} *     TreeNode(int val) { this.val = val; } *     TreeNode(int val, TreeNode left, TreeNode right) { *         this.val = val; *         this.left = left; *         this.right = right; *     } * } */class BSTIterator {
       private List
   
     list;    private int index;        public BSTIterator(TreeNode root) {
           this.list = new ArrayList<>();        this.index = -1;        this.inOrder(root);    }    private void inOrder(TreeNode root) {
           if(root == null) {
               return;        }        inOrder(root.left);        list.add(root.val);        inOrder(root.right);    }        public int next() {
           return this.list.get(++index);    }        public boolean hasNext() {
           return (this.index + 1) < list.size();    }}/** * Your BSTIterator object will be instantiated and called as such: * BSTIterator obj = new BSTIterator(root); * int param_1 = obj.next(); * boolean param_2 = obj.hasNext(); */
   

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