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_tree_level_order_traversal
+
+Given the `root`of a binary tree, return *the level order traversal of its nodes' values*
+. (i.e., from left to right, level by level).
+
+## Examples
+
+**Example 1:**
+
+![https://assets.leetcode.com/uploads/2021/02/19/tree1.jpg](https://assets.leetcode.com/uploads/2021/02/19/tree1.jpg)
+
+```
+Input: root = [3,9,20,null,null,15,7]
+Output: [[3],[9,20],[15,7]]
+
+```
+
+**Example 2:**
+
+```
+Input: root = [1]
+Output: [[1]]
+
+```
+
+**Example 3:**
+
+```
+Input: root = []
+Output: []
+
+```
+
+**Constraints:**
+
+- The number of nodes in the tree is in the range `[0, 2000]`.
+- `-1000 <= Node.val <= 1000`
+
+## 解析
+
+題目給定一個二元樹根結點 root。
+
+要求實作一個演算法根據 level order 來尋訪二元樹，並回傳每個 level的結構
+
+這題跟 [199. Binary Tree Right Side View](https://www.notion.so/199-Binary-Tree-Right-Side-View-266ca3b629444b019b41652423f51175) 一樣需要用 Breadth First Search 演算法來實作
+
+使用一個 queue 來儲存每個 level 的所有 node 
+
+每次都把這個 queue 的 level 紀錄下來及為所求
+
+如下圖
+
+![](https://i.imgur.com/LBC8UmF.png)
+
+這樣等到 queue 為空時，整個tree 都走訪結束
+
+因此時間複雜度是 O(n) ，空間複雜度也是 O(n)
+
+## 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;
+   *     }
+   * }
+   */
+  public List<List<Integer>> levelOrder(TreeNode root) {
+    List<List<Integer>> result = new ArrayList<>();
+    Queue<TreeNode> queue = new LinkedList<>();
+    queue.add(root);
+    while(queue.size() > 0) {
+      int levelSize = queue.size();
+      List<Integer> level = new ArrayList<>();
+      for (int count = 0; count < levelSize; count++) {
+        TreeNode node = queue.poll();
+        if (node != null) {
+          level.add(node.val);
+          queue.add(node.left);
+          queue.add(node.right);
+        }
+      }
+      if (level.size() > 0) {
+        result.add(level);
+      }
+    }
+    return result;
+  }
+}
+```
+## 困難點
+
+1. 理解二元樹 level order traversal的意思
+2. 理解怎麼去做 level order traversal
+
+## Solve Point
+
+- [x]  Understand what problem would like 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()
+}

.idea/vcs.xml

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+distributionBase=GRADLE_USER_HOME
+distributionPath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-7.4-bin.zip
+zipStoreBase=GRADLE_USER_HOME
+zipStorePath=wrapper/dists