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前言

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可以直接中序遍历两个 BST 得到两个有序数组，然后把这两个有序数组合并，这个思路简单，但是空间复杂度会高一些

一、力扣449. 序列化和反序列化二叉搜索树

/*** Definition for a binary tree node.* public class TreeNode {*     int val;*     TreeNode left;*     TreeNode right;*     TreeNode(int x) { val = x; }* }*/
public class Codec {StringBuilder sb = new StringBuilder();// Encodes a tree to a single string.public String serialize(TreeNode root) {fun(root);return sb.toString();}// Decodes your encoded data to tree.public TreeNode deserialize(String data) {if(data.isEmpty()){return null;}LinkedList<Integer> list = new LinkedList<>();for(String s : data.split(",")){list.add(Integer.parseInt(s));}return fun(list,Integer.MIN_VALUE, Integer.MAX_VALUE);}public TreeNode fun(LinkedList<Integer> list, int min, int max){if(list.isEmpty()){return null;}int temp = list.getFirst();if(temp > max || temp < min){return null;}list.removeFirst();TreeNode cur = new TreeNode(temp);cur.left = fun(list, min, temp);cur.right = fun(list, temp, max);return cur;}public void fun(TreeNode root){if(root == null){return;}sb.append(root.val).append(",");fun(root.left);fun(root.right);}
}// Your Codec object will be instantiated and called as such:
// Codec ser = new Codec();
// Codec deser = new Codec();
// String tree = ser.serialize(root);
// TreeNode ans = deser.deserialize(tree);
// return ans;

二、力扣173. 二叉搜索树迭代器

/*** 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 {List<Integer> list = new ArrayList<>();int index = 0;public BSTIterator(TreeNode root) {fun(root);}public int next() {int cur = list.get(index);index ++;return cur;}public boolean hasNext() {if(index < list.size()){return true;}return false;}public void fun(TreeNode root){if(root == null){return ;}fun(root.left);list.add(root.val);fun(root.right);}
}/*** 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();*/