LeetCode Java Base Knowledge Sheetcheet (Continuously Updating...)

🔥 LeetCode Java Base Knowledge Summary

(Below contents are generated with LLM tools.) When solving LeetCode problems in Java, you need to be familiar with string operations, arrays, collections, algorithms, and data structures. Below is a list of essential Java concepts.

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📌 1️⃣ String Operations

1.1 Basic String Operations

Method	Function	Example
s.length()	Get string length	"abc".length() → 3
s.charAt(i)	Get character at index i	"abc".charAt(1) → 'b'
s.equals(t)	Compare two strings	"abc".equals("abc") → true
s.compareTo(t)	Compare strings lexicographically	"abc".compareTo("bcd") → -1

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1.2 Splitting & Joining Strings

Method	Function	Example
s.split(" ")	Split string into String[] by whitespace	"a b c".split(" ") → ["a", "b", "c"]
String.join(" ", arr)	Join string array with a separator	String.join("-", ["a", "b", "c"]) → "a-b-c"
s.toCharArray()	Convert string to char[]	"abc".toCharArray() → ['a', 'b', 'c']

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1.3 Modifying Strings

Method	Function	Example
s.trim()	Remove leading & trailing spaces	" abc ".trim() → "abc"
s.replace("a", "x")	Replace all occurrences of a character	"abc".replace("a", "x") → "xbc"
s.replaceAll("\\s+", " ")	Replace multiple spaces with a single space	"a b".replaceAll("\\s+", " ") → "a b"

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1.4 Reversing a String

Method 1: built-in methods

String s = "hello";
String reversed = new StringBuilder(s).reverse().toString();
System.out.println(reversed); // "olleh"

Method 2: double pointers

char[] arr = s.toCharArray();
int left = 0, right = arr.length - 1;
while (left < right) {
    char temp = arr[left];
    arr[left++] = arr[right];
    arr[right--] = temp;
}
System.out.println(new String(arr)); // "olleh"

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📌 2️⃣ Array Operations

2.1 Basic Array Operations

Method	Function	Example
int[] arr = new int[10]	Initialize an array	[0,0,0,0,0,0,0,0,0,0]
Arrays.fill(arr, 1)	Fill the array with a value	[1,1,1,1,1]
Arrays.toString(arr)	Convert the array into a string	"[1,1,1,1,1]"

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2.2 Sorting

Method	Function	Example
Arrays.sort(arr)	Sort an array in ascending order	[3,2,1] → [1,2,3]
Arrays.sort(arr, Collections.reverseOrder())	Sort in descending order	[3,2,1] → [3,2,1]

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2.3 Copying an Array

int[] original = {1, 2, 3};
int[] copy = Arrays.copyOf(original, 5);
System.out.println(Arrays.toString(copy)); // [1,2,3,0,0]

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📌 3️⃣ Collection Framework

3.1 ArrayList (Dynamic Array)

List<Integer> list = new ArrayList<>();
list.add(5);
System.out.println(list.get(0)); // 5

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3.2 HashSet (Remove Duplicates)

Set<Integer> set = new HashSet<>();
set.add(5);
System.out.println(set.contains(5)); // true

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3.3 HashMap (Key-Value Pairs)

Map<Integer, String> map = new HashMap<>();
map.put(1, "one");
System.out.println(map.get(1)); // "one"

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🚀 Summary: Essential Java Knowledge for LeetCode

Category	Key Knowledge
String	split(), toCharArray(), StringBuilder
Array	Arrays.sort(), Arrays.fill()
Collections	ArrayList, HashMap, HashSet
Queue	Queue, Deque
Heap	PriorityQueue
Sorting	Comparator, Collections.sort()
Dynamic Programming	dp[], dp[][]

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