Leetcode - Tips

1.java The method by which the program obtains console input
import java.util.Scanner;// Pilot Kit
Scanner sc = new Scanner(System.in);
String str = sc.next();// The input obtained is String type
int a1 = sc.nextInt();// The input obtained is int type
double a2= sc.nextDouble();// The input obtained is double type
 

2.Java Build a stack

Stack.peek() and Stack.pop() The difference between

Stack<TreeNode> stack = new Stack<>();  //

1. Stack.peek()
peek() Function returns the element at the top of the stack , But do not pop up the top element of the stack .

2. Stack.pop()
pop() Function returns the element at the top of the stack , And push the top element of the stack out of the stack .

3.Java Create a queue

Use ：Deque Interface

 Deque<Integer> test = new LinkedList<>();

 int currentLevelSize = queue.size();  //  The queue length 

public class Main {
    public static void main(String[] args) {
        //add() and remove() Method throws an exception when it fails ( Not recommended )
        Queue<String> queue = new LinkedList<String>();
        // Additive elements 
        queue.offer("a");
        queue.offer("b");
        queue.offer("c");
        queue.offer("d");
        queue.offer("e");
        for(String q : queue){
            System.out.println(q);
        }
        System.out.println("===");
        System.out.println("poll="+queue.poll()); // Returns the first element , And delete... In the queue 
        for(String q : queue){
            System.out.println(q);
        }
        System.out.println("===");
        System.out.println("element="+queue.element()); // Returns the first element  
        for(String q : queue){
            System.out.println(q);
        }
        System.out.println("===");
        System.out.println("peek="+queue.peek()); // Returns the first element  
        for(String q : queue){
            System.out.println(q);
        }
    }
}

a
b
c
d
e
===
poll=a
b
c
d
e
===
element=b
b
c
d
e
===
peek=b
b
c
d
e

Deque It's a two terminal queue interface , Inherited from Queue Interface ,Deque The implementation class is LinkedList、ArrayDeque、LinkedBlockingDeque, among LinkedList Is the most commonly used .

4.LinkedList usage

  Reference resources ：

LinkedList Usage details _ Han family boss's blog -CSDN Blog _linkedlist

5.ArrayList usage

3-1.ArrayList How to add

Method 1： Go to... In order ArrayList Add data to .

usage ： take a Add to list in ,list.add("a");

 3-2 ArrayList Delete method of

Method 1： Delete individual data by location

usage ： take list pass the civil examinations 2 Data deletion  

list.remove(2);

Be careful ： Location slave 0 Start calculating (0、1、2、3...)

Method 2： Delete single data according to content

usage ：

take list Data in "d" Delete

list.remove("d");

Be careful ： about int,String,char Such original type data can be deleted , But for complex objects , For example, I wrote it myself User class 、Person Class object , Need to rewrite equals Method , be responsible for remove Method cannot match delete .

Reference resources ：

Java Of ArrayList usage _Fighting_ Chuxin's blog -CSDN Blog _arraylist usage

Middle order and post order sequence , Build trees

 Map<String, Integer> map = new HashMap<String, Integer>();
        map.put(" Zhang San ", 18);
        map.put(" Li Si ", 28);
        map.put(" Wang Wu ", 38);
        map.put(" Zhao Liu ", 48);
        //1. obtain map A collection of entry  aggregate 
        Set<Map.Entry<String, Integer>> entries = map.entrySet();
        //2. Use iterators   Traverse this entries This Set aggregate 
        Iterator<Map.Entry<String, Integer>> it = entries.iterator();
        //3. Traverse 
        while(it.hasNext()){
            Map.Entry<String, Integer> entry = it.next();
            // One entry in   There are two properties 
            String key = entry.getKey();
            Integer value = entry.getValue();
            System.out.println(key+"="+value);
            
        }

 public class MapDemo {
    public static void main(String[] args) {
        // Define a Map aggregate   Store keys for students （ full name , Age ） It's worth the student's own home address .
        Map<Student, String> map = new HashMap<Student, String>();
        // Add data to the collection 
        map.put(new Student(" Baoqiang Wang ", 40), " Outside the Fifth Ring Road in Beijing ");
        map.put(new Student(" Nicholas Tse ", 50), " Beijing 180 Outside the ring ");
        map.put(new Student(" Ma Rong ", 45), " Shanghai Jiaotong road ");
        map.put(new Student(" Guo Degang ", 55), " Guangzhou Deyun society ");
        map.put(new Student(" Ma Rong ", 45), " My house ");
        //map Judge whether the key is repeated or not , It's through hashCode and equals Method 
        // If   I want a student's name and age to be the same   I think it's the same student 
        // Ergodic set  keySet entrySet
        //1. obtain entry Set 
        Set<Map.Entry<Student, String>> entries = map.entrySet();
        //2. Iterator traversal  foreach
        for (Map.Entry<Student, String> entry : entries) {
            Student key = entry.getKey();
            String value = entry.getValue();
            System.out.println(key+"="+value);
        }
    }
}