Java面试常用算法api速刷

数组

int[] arr1 = new int[5];

int[] arr2 = {1, 2, 3, 4, 5};

int[] arr3 = new int[]{3, 1, 2, 4, 5};

int[][] matrix1 = new int[3][3];

int[][] matrix2 = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};

int[][] matrix3 = new int[][]{{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};

arr**.length**

Arrays.fill (arr1, 1);

Arrays.sort (arr3);

Arrays.copyOfRange (arr3, 1, 3);

List<Integer> list = Arrays.asList (arr); //list指向原数组位置

List<Integer> list = new ArrayList<>(Arrays.asList(1, 2, 3)); //List指向新数组位置

Integer[] newArray = list2.toArray(new Integer[0]);

String

String str1 = "Hello";

String str2 = new String("World");

str1.length ()

str2.charAt (0)

str1.equals (str2)

str1.substring (1, 3) //起始位置，最终位置，左闭右开

str1.indexOf ("ll")

str1.lastIndexOf ("l")

str1.toUpperCase ()

str1.toLowerCase ()

str1.replace ("l", "p")

String[] split = "a,b,c".split (",");

str.trim()

char[] charArray = str1**.toCharArray()**;

StringBuilder

StringBuilder sb = new StringBuilder();

sb.append (str1).append(str2);

sb.reverse().toString()

sb.insert (4, "insert");

sb.replace (4, 10, "java"); //起始位置，最终位置，左闭右开

sb.reverse ();

sb.substring (1,3) //起始位置，最终位置，左闭右开

sb.setCharAt (1, 'T');

sb.length ()

sb.charAt (1)

sb.toString()

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集合通用：

size()

clear()

isEmpty()

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ArrayList

List<String> arrayList = new ArrayList<>();

List<String> arrayList1 = new ArrayList<>(Arrays.asList("1", "2", "3"));

arrayList.add("4");

arrayList.get(0);

arrayList.set(0, "10");

arrayList.remove("2");

arrayList.remove(0);

arrayList.contains("3")

arrayList.size()

arrayList1.clear();

// 迭代器遍历List

Iterator<String> iterator = linkedList1.iterator();

while (iterator.hasNext())

System.out.println(iterator.next());

HashSet

Set<String> hashSet = new HashSet<>();

Set<String> hashSet1 = new HashSet<>(Arrays.asList("1", "2", "3"));

hashSet1.add("4");

hashSet1.remove("4");

hashSet1.contains("3")

hashSet1.size()

hashSet1.clear();

HashMap

Map<Integer, String> hashMap = new HashMap<>();

hashMap.put(1, "1");

hashMap.put(2, "2");

hashMap.remove(2);

hashMap.get(1);

// 遍历键

for (Integer key : hashMap.keySet()) {

System.out.println("key: " + key);

}

// 遍历值

for (String val : hashMap.values()) {

System.out.println("value: " + val);

}

ArrayDeque

Deque<Integer> arrayDeque = new ArrayDeque<>();

arrayDeque.peek();

arrayDeque.offerFirst(2);

arrayDeque.offerLast(12);

arrayDeque.offer(13); //跟末尾插入一样

arrayDeque.pollFirst();

arrayDeque.pollLast();

arrayDeque.poll(); //跟末尾弹出一样

//当栈用

arrayDeque.peek();

arrayDeque.pop();

arrayDeque.push();

PriorityQueue

PriorityQueue<Integer> priorityQueue = new PriorityQueue<>();

priorityQueue.offer(1);

priorityQueue.poll()

priorityQueue.peek()

PriorityQueue<Integer> priorityQueue1 = new PriorityQueue<>((a, b) -> b - a);

priorityQueue1.offer(1);

Scanner工具类

Scanner scanner = new Scanner(System.in);

int num = scanner.nextInt();

String str = scanner.next(); // 读取下一个字符串（到空格为止）

scanner.nextLine(); // 读取换行符，避免影响下一次读取

String str = scanner.nextLine(); // 获取用户输入的字符串

scanner.close(); // 关闭 Scanner 对象

• boolean hasNext()：检查输入流是否还有下一个标记。
• boolean hasNextLine()：检查输入流是否还有下一行。