Data Structure

HashMap

https://docs.oracle.com/javase/tutorial/collections/interfaces/map.html

HashMap 的两种遍历方式
第一种

  Map map = new HashMap();
  Iterator iter = map.entrySet().iterator();
  while (iter.hasNext()) {
      Map.Entry entry = (Map.Entry) iter.next();
      Object key = entry.getKey();
      Object val = entry.getValue();
  }

效率高,以后一定要使用此种方式!

第二种

  Map map = new HashMap();
  Iterator iter = map.keySet().iterator();
      while (iter.hasNext()) {
      Object key = iter.next();
      Object val = map.get(key);
  }

效率低,以后尽量少使用!

Map Traverse:

Map<String, Integer> map = new HashMap();
// Keys
for(String str : map.keySet()){
    Integer value = map.get(str);
}

// Map.Entry
for (Map.Entry<K, V> e : map.entrySet())
    System.out.println(e.getKey() + ": " + e.getValue());

The Collection view methods allow a Map to be viewed as a Collection in these three ways:

  • keySet — the Set of keys contained in the Map.
  • values — The Collection of values contained in the Map. This Collection is not a Set, because multiple keys can map to the same value.
  • entrySet — the Set of key-value pairs contained in the Map. The Map interface provides a small nested interface called Map.Entry, the type of the elements in this Set.

Differences Between HashMap Vs HashSet In Java

https://javaconceptoftheday.com/differences-between-hashmap-vs-hashset-in-java/

Union Find (Disjoint Set)

并查集: 一种用来解决集合查询合并的数据结构 支持O(1) find / O(1) union

并查集可以干什么?

  1. 判断在不在同一个集合中。
    • find 操作
  2. 关于集合合并
    • union 操作

并查集的操作

  1. 查询 Find (递归? 非递归?)

模板代码

HashMap<Integer, Integer> father = new HashMap<Integer, Integer>();

int find(int x) {
    int parent = x;
    while (parent ! = father.get(parent)) {
        parent = father.get(parent);
    }
    return parent;
}
  1. 合并 Union

老大哥之间合并
跟小弟没关系

HashMap<Integer, Integer> father = new HashMap<Integer, Integer>();

void union(int x, int y) {
    int fa_x = find(x);
    int fa_y = find(y);
    if (fa_x != fa_y) {
        father.put(fa_x, fa_y);
    }
}

并查集完整模板

class UnionFind {
    UnionFind() {}
    HashMap<Integer, Integer> father = new HashMap<Integer, Integer>();
    int find(int x) {
        int parent = x;
        while (parent ! = father.get(parent)) {
            parent = father.get(parent);
        }
        return parent;
    }
    void union(int x, int y) {
        int fa_x = find(x);
        int fa_y = find(y);
        if (fa_x != fa_y) {
            father.put(fa_x, fa_y);
        }
    }
}

Trie

Linked List vs ArrayList

LinkedList and ArrayList are two different implementations of the List interface. LinkedList implements it with a doubly-linked list. ArrayList implements it with a dynamically re-sizing array.

LinkedList<E> allows for constant-time insertions or removals using iterators, but only sequential access of elements.

ArrayList<E>, on the other hand, allow fast random read access, so you can grab any element in constant time.

ArrayList LinkedList
1) ArrayList internally uses a dynamic array to store the elements. LinkedList internally uses a doubly linked list to store the elements.
2) Manipulation with ArrayList is slow because it internally uses an array. If any element is removed from the array, all the bits are shifted in memory. Manipulation with LinkedList is faster than ArrayList because it uses a doubly linked list, so no bit shifting is required in memory.
3) An ArrayList class can act as a list only because it implements List only. LinkedList class can act as a list and queue both because it implements List and Deque interfaces.
4) ArrayList is better for storing and accessing data. LinkedList is better for manipulating data.

Heap

Heap

A min-heap is a binary tree such that

  • the data contained in each node is less than (or equal to) the data in that node’s children.
  • the binary tree is complete

A max-heap is a binary tree such that

  • the data contained in each node is greater than (or equal to) the data in that node’s children.
  • the binary tree is complete

Sift Up

void siftup(int id) {
    while (parent(id) > -1) {
        int parentId = parent(id);
        if (comparesmall(heap.get(parentId), heap.get(id)) == true) {
            break;
        } else {
            swap(id, parentId);
        }
        id = parentId;
    }
}

Sift Down

void siftdown(int id) {
    while (lson(id) < heap.size()) {
        int leftId = lson(id);
        int rightId = rson(id);
        int son;
        if (rightId >= heap.size() || (comparesmall(heap.get(leftId), heap.get(rightId)) == true)) {
            son = leftId;
        } else {
            son = rightId;
        }

        if (comparesmall(heap.get(id), heap.get(son)) == true) {
            break;
        } else {
            swap(id, son);
        }
        id = son;
    }
}

Resources

Stack

Queue

ArrayDeque / Deque

ArrayDeque

  • It’s not thread-safe
  • Null elements are not accepted
  • Works significantly faster than the synchronized Stack
  • Is a faster queue than LinkedList due to the better locality of reference
  • Most operations have amortized constant time complexity
  • An Iterator returned by an ArrayDeque is fail-fast
  • ArrayDeque automatically doubles the size of an array when head and tail pointer meets each other while adding an element

An ArrayDeque implementation can be used as a Stack (Last-In-First-Out) or a Queue(First-In-First-Out).

In Java Docs for ArrayDeque:

This class is likely to be faster than Stack when used as a stack, and faster than LinkedList when used as a queue.

Deque Interface:

https://docs.oracle.com/javase/8/docs/api/java/util/Deque.html

Arrays

java.util.Arrays

https://www.geeksforgeeks.org/array-class-in-java/

Methods

1.static List asList(T… a): This method returns a fixed-size list backed by the specified array.

// Java program to demonstrate 
// Array.asList() method 

import java.util.Arrays; 

public class Main { 
    public static void main(String[] args) 
    { 

        // Get the Array 
        int intArr[] = { 10, 20, 15, 22, 35 }; 

        // To convert the elements as List 
        System.out.println("Integer Array as List: "
                        + Arrays.asList(intArr)); 
    } 
}
  1. static int binarySearch(elementToBeSearched): These methods searches for the specified element in the array with the help of Binary Search algorithm.
// Java program to demonstrate 
// Array.binarySearch() method 

import java.util.Arrays; 

public class Main { 
    public static void main(String[] args) 
    { 

        // Get the Array 
        int intArr[] = { 10, 20, 15, 22, 35 }; 

        Arrays.sort(intArr); 

        int intKey = 22; 

        System.out.println(intKey 
                        + " found at index = "
                        + Arrays 
                                .binarySearch(intArr, intKey)); 
    } 
}
  1. copyOf(originalArray, newLength): This method copies the specified array, truncating or padding with the default value (if necessary) so the copy has the specified length.
// Java program to demonstrate 
// Array.copyOf() method 

import java.util.Arrays; 

public class Main { 
    public static void main(String[] args) 
    { 

        // Get the Array 
        int intArr[] = { 10, 20, 15, 22, 35 }; 

        // To print the elements in one line 
        System.out.println("Integer Array: "
                        + Arrays.toString(intArr)); 

        System.out.println("\nNew Arrays by copyOf:\n"); 

        System.out.println("Integer Array: "
                        + Arrays.toString( 
                                Arrays.copyOf(intArr, 10))); 
    } 
}
  1. fill(originalArray, fillValue): This method assigns this fillValue to each index of this array.
// Java program to demonstrate 
// Array.fill() method 

import java.util.Arrays; 

public class Main { 
    public static void main(String[] args) 
    { 

        // Get the Arrays 
        int intArr[] = { 10, 20, 15, 22, 35 }; 

        int intKey = 22; 

        Arrays.fill(intArr, intKey); 

        // To fill the arrays 
        System.out.println("Integer Array on filling: "
                        + Arrays.toString(intArr)); 
    } 
}

TreeSet / TreeMap

TreeMap

https://www.baeldung.com/java-treemap

Default Sorting in TreeMap

@Test
public void givenTreeMap_whenOrdersEntriesNaturally_thenCorrect() {
    TreeMap<Integer, String> map = new TreeMap<>();
    map.put(3, "val");
    map.put(2, "val");
    map.put(1, "val");
    map.put(5, "val");
    map.put(4, "val");

    assertEquals("[1, 2, 3, 4, 5]", map.keySet().toString());
}

Custom Sorting in TreeMap

public void givenTreeMap_whenOrdersEntriesByComparator_thenCorrect() {
    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());
}

TreeSet

https://www.baeldung.com/java-tree-set

In this implementation, objects are sorted and stored in ascending order according to their natural order. The _TreeSet _uses a self-balancing binary search tree, more specifically a _Red-Black _tree.

Operations like add, remove and contains (search) take O(log n) time while operations like printing n elements in sorted order require O(n) time.

Set<String> treeSet = new TreeSet<>();

Comparator

Set<String> treeSet = new TreeSet<>(Comparator.comparing(String::length));

Although TreeSet isn’t thread-safe, it can be synchronized externally using the Collections.synchronizedSet() wrapper:

 Set<String> syncTreeSet = Collections.synchronizedSet(treeSet);

TreeSet add()

TreeSet contains()

TreeSet remove()

TreeSet & TreeMap

The TreeSet internally depends on a backing NavigableMap which gets initialized with an instance of TreeMap when an instance of the TreeSet is created:

public TreeSet() {
    this(new TreeMap<E,Object>());
}

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