二叉树前序（中序）遍历，求二叉树

class TreeNode {
     int val;
     TreeNode left;
     TreeNode right;
     TreeNode(int x) { val = x; }
 }
class Solution {
    private Map<Integer,Integer> mp;

    /**
     * 递归遍历
     * @param preorder
     * @param inorder
     * @param preleft 子树的左边界
     * @param preright 子树右边界
     * @param ordleft 子树左边界
     * @param ordright 子树有边界
     * @return
     */
    private TreeNode retuTree(int [] preorder,int [] inorder,int preleft,int preright,
                              int ordleft,int ordright){
        if(preleft>preright)return null;

        TreeNode node = new TreeNode(preorder[preleft]);
        //左子树长度
        int len = (Integer)mp.get(preorder[preleft])-ordleft;
        if(preleft==preright)return node;
        
        node.left = retuTree(preorder,inorder,preleft+1,preleft+len,ordleft,ordleft+len);
        node.right = retuTree(preorder,inorder,preleft+len+1,preright,ordleft+len+1,ordright);
        return node;
    }

    /**
     * 根据二叉树前序和中序遍历，返回完整二叉树
     * @param preorder 前序遍历序列
     * @param inorder 中序遍历序列
     * @return 返回二叉树根节点
     */
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        mp = new HashMap<Integer,Integer>();
        int n=preorder.length;
        if(n<1)return null;
        for(int i=0;i<n;i++){
            mp.put(inorder[i],i);
        }
        return retuTree(preorder,inorder,0,n-1,0,n-1);
    }
}

题目来源：https://leetcode.cn/problems/zhong-jian-er-cha-shu-lcof/