从前序遍历和中序遍历构建二叉树

class Solution {    public HashMap
     
       map = new HashMap<>();    public TreeNode buildTree(int[] preorder, int[] inorder) {        int len = preorder.length;        for(int i=0;i
      
       end) {            return null;        }          TreeNode root = new TreeNode(preorder[first]);        int middle = map.get(preorder[first]);        root.left = buildTree2(preorder,inorder,start,middle-1,first+1);        root.right = buildTree2(preorder,inorder,middle+1,end,first+(middle-start)+1);        return root;    }}