# 106.Construct-Binary-Tree-from-Inorder-and-Postorder-Traversal

## 106. Construct Binary Tree from Inorder and Postorder Traversal

## 题目地址

<https://leetcode.com/problems/construct-binary-tree-from-inorder-and-postorder-traversal/>

## 题目描述

```
Given inorder and postorder traversal of a tree, construct the binary tree.

Note:
You may assume that duplicates do not exist in the tree.

For example, given

inorder = [9,3,15,20,7]
postorder = [9,15,7,20,3]
Return the following binary tree:

    3
   / \
  9  20
    /  \
   15   7
```

## 代码

### Approach #1 Recursion

```java
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
  int post_idx;
  int[] postorder;
  HashMap<Integer, Integer> idx_map = new HashMap();
  public TreeNode buildTree(int[] inorder, int[] postorder) {
        this.postorder = postorder;
    post_idx = postorder.length - 1;

    int idx = 0;
    for (Integer val: inorder) {
      idx_map.put(val, idx++);
    }
    return helper(0, inorder.length - 1);
  }

  public TreeNode helper(int in_left, int in_right) {
    if (in_left > in_right) { // inorder的作用
      return null;
    }

    int root_val = postorder[post_idx];
    TreeNode root = new TreeNode(root_val);

    int index = idx_map.get(root_val); // inorder的作用

    post_idx--;
    root.left = helper(in_left, index - 1);
    root.right = helper(index + 1, in_right);

    return root;
  }
}
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://wentao-shao.gitbook.io/leetcode/binary-tree/106.construct-binary-tree-from-inorder-and-postorder-traversal.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.