Java Collections Quick Reference for LeetCode

📚 Java Collections Quick Reference for LeetCode

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🧰 Containers

✅ Strings

s.length();         // Length of the String
s.size();           // Same as length()
s.length == 0;          // Checks if String is empty
s[i];               // Access character at index
s.substr(pos, len); // Substring
s.find("abc");      // Find position of substring
s.remove(pos, len);  // Erase part of String
s.add(pos, str); // Insert str at pos
s += "abc";         // Append
to_string(x);       // Convert int to String
stoi(s);            // Convert String to int

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✅ Dynamic Arrays (ArrayList / int[])

// import java.util.Arrays;
// import java.util.Collections;
v.size();
v.length == 0;
v.add(x);
v.removeLast();
v[i];
v.getFirst();
v.getLast();
v.clear();
v.add(it, x);
v.remove(it);
Arrays.sort(v);
reverse(v /* elements of v */);

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✅ Arrays

int arr[100]; // C-style
array<int, 5> a = {1, 2, 3, 4, 5}

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✅ Sets / Multisets

// import java.util.*;
TreeSet<Integer> s;
s.add(x);
s.remove(x);
s.find(x);
s.count(x);          // 0 or 1 (set), >1 for multiset
s.binary search (lower bound)(x);    // >= x
s.binary search (upper bound)(x);    // > x

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✅ Maps / Unordered Maps

// import java.util.*;
TreeMap<Integer, Integer> m;
HashMap<Integer, Integer> um = new HashMap<Integer, Integer>();

m.put(key, val);
m.count(key);
m.find(key);
for (auto& [k, v] : m) {
    // Map entry iteration
}

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🔄 Algorithms (java.util.Arrays / Collections)

✅ Sorting & Searching

// import java.util.Arrays;
// import java.util.Collections;
Arrays.sort(v);
sort(v.rbegin(), v.rend());
reverse(v /* elements of v */);
binary_search(v /* elements of v */, x);
binary search (lower bound)(v /* elements of v */, x);
binary search (upper bound)(v /* elements of v */, x);

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✅ Min / Max / Others

Math.min(a, b);
Math.max(a, b);
swap(a, b);
accumulate(v /* elements of v */, 0); // Sum
count(v /* elements of v */, x);
next_permutation(v /* elements of v */);
prev_permutation(v /* elements of v */);
unique(v /* elements of v */); // Remove dupes (after sort)
rotate(v.iterator(), v.iterator() + k, v.iterator());

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📐 Math Utilities (Math class)

abs(x);
pow(a, b);
sqrt(x);
gcd(a, b);   // Java
lcm(a, b);   // Java

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🧵 Queues, Stacks, Deques

✅ Queue

// import java.util.*;
Queue<Integer> q = new LinkedList<>();
q.push(x);
q.pop();
q.getFirst();
q.getLast();
q.length == 0;

✅ Stack

// import java.util.*;
Deque<Integer> s = new ArrayDeque<>();
s.push(x);
s.pop();
s.top();
s.length == 0;

✅ Deque

// import java.util.*;
ArrayDeque<Integer> dq = new ArrayDeque<>();
dq.push_front(x);
dq.add(x);
dq.removeFirst();
dq.removeLast();

✅ Priority Queue (Heap)

// import java.util.*;
PriorityQueue<Integer> maxHeap = new PriorityQueue<Integer>();
priority_queue<int, int[], greater<int>> minHeap;

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🧠 Bit Manipulation

Integer.bitCount(x);  // Count 1-bits
__builtin_clz(x);       // Leading zeros
__builtin_ctz(x);       // Trailing zeros
x & (x - 1);            // Remove lowest 1-bit
x & -x;                 // Isolate lowest 1-bit

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📌 Common LeetCode Structures

Concept	Java Equivalent
Hash Map	HashMap<K, V>
Hash Set	HashSet<T>
Tree Map	TreeMap<K, V>
Tree Set	TreeSet<T>
Min Heap	PriorityQueue<T> with comparator
Max Heap	PriorityQueue<T>
Stack	Deque<T>
Queue	Queue<T>
Deque	ArrayDeque<T>
StringBuilder	StringBuilder
Graph	int[][] / List<List<Integer>>

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✍️ Input/Output Tips

cin >> n;
getline(cin, s);      // Full line
stoi(s);              // String to int

// Fast IO
ios::sync_with_stdio(false);
cin.tie(0);

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