init commit

Author: yuanyu90221

.github/workflows/java.yml

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+name: Java
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+      - name: Setup JDK 11
+        uses: actions/setup-java@v3
+        with:
+          distribution: 'adopt'
+          java-version: '11'
+      - name: Grant execution permission for gradlew
+        run: chmod +x gradlew
+      - name: Cache Gradle packages
+        uses: actions/cache@v3
+        with:
+          path: ~/.gradle/caches
+          key: ${{ runner.os }}-gradle-${{ github.event.repository.name }}-${{ hashFiles('**/*.gradle')}}
+          restore-keys: ${{ runner.os }}-gradle-${{ github.event.repository.name }}
+      - name: Build with Gradlew
+        run: ./gradlew build
+      - name: Test with Gradlew
+        run: ./gradlew :test --tests "SolutionTest"

.gitignore

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+.gradle
+build/
+!gradle/wrapper/gradle-wrapper.jar
+!**/src/main/**/build/
+!**/src/test/**/build/
+
+### IntelliJ IDEA ###
+.idea/modules.xml
+.idea/jarRepositories.xml
+.idea/compiler.xml
+.idea/libraries/
+*.iws
+*.iml
+*.ipr
+out/
+!**/src/main/**/out/
+!**/src/test/**/out/
+
+### Eclipse ###
+.apt_generated
+.classpath
+.factorypath
+.project
+.settings
+.springBeans
+.sts4-cache
+bin/
+!**/src/main/**/bin/
+!**/src/test/**/bin/
+
+### NetBeans ###
+/nbproject/private/
+/nbbuild/
+/dist/
+/nbdist/
+/.nb-gradle/
+
+### VS Code ###
+.vscode/
+
+### Mac OS ###
+.DS_Store

.husky/pre-commit

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+#!/bin/bash
+./gradlew :test --tests "SolutionTest"

.idea/gradle.xml

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+# java_build_binary_tree_from_preOrder_inOrder
+
+Given two integer arrays `preorder` and `inorder` where `preorder` is the preorder traversal of a binary tree and `inorder` is the inorder traversal of the same tree, construct and return *the binary tree*.
+
+## Examples
+
+**Example 1:**
+
+![https://assets.leetcode.com/uploads/2021/02/19/tree.jpg](https://assets.leetcode.com/uploads/2021/02/19/tree.jpg)
+
+```
+Input: preorder = [3,9,20,15,7], inorder = [9,3,15,20,7]
+Output: [3,9,20,null,null,15,7]
+
+```
+
+**Example 2:**
+
+```
+Input: preorder = [-1], inorder = [-1]
+Output: [-1]
+
+```
+
+**Constraints:**
+
+- `1 <= preorder.length <= 3000`
+- `inorder.length == preorder.length`
+- `3000 <= preorder[i], inorder[i] <= 3000`
+- `preorder` and `inorder` consist of **unique** values.
+- Each value of `inorder` also appears in `preorder`.
+- `preorder` is **guaranteed** to be the preorder traversal of the tree.
+- `inorder` is **guaranteed** to be the inorder traversal of the tree.
+
+## 解析
+
+題目以 Pre-Order Traversal 與 In-Order Traversal 給出兩個整數陣列
+
+求實作一個演算法建立出這棵二元樹
+
+Pre-Order Traversal 特性會有以下走訪順序如下：
+
+二元樹root , root.Left 子樹, root.Right 子樹
+
+In-Order Traversal 特性會有以下走訪順序如下：
+
+root.Left 子樹, 二元樹root, root.Right 子樹
+
+觀察下圖
+
+![](https://i.imgur.com/KYVbaXL.png)
+
+可以發現
+
+每個 pre-order的點先遇到的都是子樹的跟結點
+
+透過 pre-order的點去找到在 in-order的位置剛好可以把左右子數分開
+
+所以可以透過遞迴關係來建立二元樹
+
+## 程式碼
+```java
+class Solution {
+  /**
+   * 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;
+   *     }
+   * }
+   */
+  int preorderIdx;
+  public TreeNode buildTree(int[] preorder, int[] inorder) {
+    preorderIdx = 0;
+    HashMap<Integer, Integer> inorderIdxMap = new HashMap<>();
+    for (int idx = 0; idx < inorder.length; idx++) {
+      inorderIdxMap.put(inorder[idx], idx);
+    }
+    return arrayToTree(preorder, 0, preorder.length - 1, inorderIdxMap);
+  }
+  public TreeNode arrayToTree(int[] preorder, int left, int right, HashMap<Integer, Integer> inorderIdxMap) {
+    if (left > right) {
+      return null;
+    }
+    int rootValue = preorder[preorderIdx];
+    preorderIdx++;
+    TreeNode root = new TreeNode(rootValue);
+    root.left = arrayToTree(preorder, left, inorderIdxMap.get(rootValue) - 1, inorderIdxMap);
+    root.right = arrayToTree(preorder,inorderIdxMap.get(rootValue) + 1, right, inorderIdxMap);
+    return root;
+  }
+}
+```
+
+## 困難點
+
+1. 看出 Pre-Order Traversal 與 In-Order Traversal 的特性
+2. 看出遞迴關係
+
+## Solve Point
+
+- [x]  Understand what problem need to solve
+- [x]  Analysis Complexity 

.idea/misc.xml

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+plugins {
+    id 'java'
+}
+
+group 'org.example'
+version '1.0-SNAPSHOT'
+
+repositories {
+    mavenCentral()
+}
+
+dependencies {
+    testImplementation 'org.junit.jupiter:junit-jupiter-api:5.8.1'
+    testRuntimeOnly 'org.junit.jupiter:junit-jupiter-engine:5.8.1'
+}
+
+test {
+    useJUnitPlatform()
+}