从前序与中序遍历序列构造二叉树

题目：从前序与中序遍历序列构造二叉树

给定一棵树的前序遍历 preorder 与中序遍历  inorder。请构造二叉树并返回其根节点。

示例 1:

Input: preorder = [3,9,20,15,7], inorder = [9,3,15,20,7]
Output: [3,9,20,null,null,15,7]

题解

class Solution {
        private int pre[];
        private int middle[];

        public TreeNode dfs(int pleft, int pright, int mleft, int mright) {
            if (pright < pleft || mright < mleft) return null;
            int val = pre[pleft];
            int y = 0;
            for (int i = mleft; i <= mright; i++) {
                if (middle[i] == val) {
                    y = i;
                    break;
                }
            }
            TreeNode root=new TreeNode(val);
            root.left=dfs(pleft+1, pleft+y-mleft, mleft, y-1);
            root.right=dfs(pleft+y-mleft+1, pright, y+1, mright);
            return root;
        }

        public TreeNode buildTree(int[] preorder, int[] inorder) {
            pre = preorder;
            middle = inorder;

            return dfs(0, preorder.length - 1, 0, inorder.length - 1);
        }
    }