Remainder

補些遺漏的部分。

HashSet

只需要存儲不重複的key，像map，但並不需要value，可以使用Set。

import java.util.*;
public class Main {
    public static void main(String[] args) {
        Set<String> set = new HashSet<>();
        System.out.println(set.add("abc")); //true
        System.out.println(set.add("xyz")); //true
        System.out.println(set.add("xyz")); //false
        System.out.println(set.contains("xyz")); //true
        System.out.println(set.contains("XYZ")); //false
        System.out.println(set.remove("hello")); //false
        System.out.println(set.size()); //2
    }
}

TreeSet

有序的Set

import java.util.*;
public class Main {
    public static void main(String[] args) {
        TreeSet ts = new TreeSet();
        ts.add("apple");
        ts.add("banana");
        ts.add("pear");
        ts.add("orange");
        System.out.println(ts);
    }
}
>
java -cp /tmp/hdkJZZyPLb Main
[apple, banana, orange, pear]

TreeMap

不像HashMap是無序的key，內部會對Key進行排序

import java.util.*;
public class Main {
    public static void main(String[] args) {
        Map<Integer,String> map = new TreeMap<>();
        map.put(2,"apple");
        map.put(3,"pear");
        map.put(1,"orange");
        for (Integer key : map.keySet()) {
            System.out.println(key);
        }
    }
}
>
1
2
3

// reverse  
TreeMap<Integer, String> map = 
      new TreeMap<>(Comparator.reverseOrder());
    map.put(3, "val");
    map.put(2, "val");
    map.put(1, "val");
    map.put(5, "val");
    map.put(4, "val");
        
    assertEquals("[5, 4, 3, 2, 1]", map.keySet().toString());

Binary Tree

import java.util.*;
class Node {
  int key;
  Node left, right;

  public Node(int item) {
      key = item;
      left = right = null;
  }
}
class BinaryTree {
  Node root;

  BinaryTree(){ root = null;}
  BinaryTree(int key){ root = new Node(key);}

  public void traverseInOrder(Node node) {
      if (node != null) {
        traverseInOrder(node.left);
        System.out.print(node.key+" ");
        traverseInOrder(node.right);
      }
  }
  
  public static void main(String[] args) {
      BinaryTree tree = new BinaryTree();
    
      tree.root = new Node(1);
      tree.root.left = new Node(2);
      tree.root.right = new Node(3);
      tree.root.left.left = new Node(4);
 
      System.out.print("\nIn order Traversal: ");
      tree.traverseInOrder(tree.root);
  }
}
>
java -cp /tmp/hdkJZZyPLb BinaryTree
In order Traversal: 4 2 1 3 

Bubble sort && Inserting sort

public class Sort {  
    static void bubbleSort(int[] arr) {  
        int temp = 0, len = arr.length;  
        for(int i=0; i < len; i++){  
             for(int j=1; j < (len-i); j++){  
                 //swap
                 if(arr[j-1] > arr[j]){  
                     temp = arr[j-1];  
                     arr[j-1] = arr[j];  
                     arr[j] = temp;  
                 }  
             }  
         }  
    }  
    public static void selectionSort(int[] arr){  
        for (int i = 0; i < arr.length - 1; i++) {  
            int index = i;  
            for (int j = i + 1; j < arr.length; j++){  
                if (arr[j] < arr[index]){  
                    index = j; 
                }  
            }  
            int smallerNumber = arr[index];   
            arr[index] = arr[i];  
            arr[i] = smallerNumber;  
        }  
    }  
    public static void main(String[] args) {  
                int arr[] ={3,60,35,2,45,320,5};  

                //System.out.println("Bubble Sort");  
                //bubbleSort(arr);
                System.out.println("Selection Sort");
                selectionSort(arr);
                 
                for(int i=0; i < arr.length; i++){  
                        System.out.print(arr[i] + " ");  
                }  
   
        }  
}  

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