LeetCode 106.从中序与后序遍历序列构造二叉树

题目：给定两个整数数组 inorder 和 postorder ，其中 inorder 是二叉树的中序遍历， postorder 是同一棵树的后序遍历，请你构造并返回这颗 二叉树 。

思路：见代码

代码：

/**
 * 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 Solution {
    public TreeNode buildTree(int[] inorder, int[] postorder) {
        int n = postorder.length;
        Map<Integer, Integer> index = new HashMap<>(n);
        for (int i = 0; i < n; i++) {
            index.put(inorder[i], i);
        }
        return dfs(postorder, 0, n, 0, n, index);
    }

    private TreeNode dfs(int[] postorder, int posL, int posR, int inL, int inR, Map<Integer, Integer> index) {
        if (posL == posR)
            return null;
        int leftSize = index.get(postorder[posR - 1]) - inL;
        TreeNode left = dfs(postorder, posL, posL + leftSize, inL, inL + leftSize, index);
        TreeNode right = dfs(postorder, posL + leftSize, posR - 1, inL + leftSize + 1, inR, index);
        return new TreeNode(postorder[posR - 1], left, right);

    }
}

性能：