Done Design-2

Problem1.java

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+// Time Complexity : O(1) amortized for push, pop, peek, and empty
+// Space Complexity : O(n)
+// Did this code successfully run on Leetcode : Yes
+// Any problem you faced while coding this : No major issues; understanding when to move elements between stacks was the key.
+
+// I used two stacks: 'in' for incoming elements and 'out' for removing elements.
+// When popping or peeking, if 'out' is empty, I move all elements from 'in' to 'out' to maintain FIFO order.
+// This ensures that each element is moved at most once between the stacks, giving amortized O(1) performance.
+
+class MyQueue {
+    Stack<Integer> in;
+    Stack<Integer> out;
+
+    public MyQueue() {
+        in = new Stack<>();
+        out = new Stack<>();
+
+    }
+
+    public void push(int x) {
+        in.push(x);
+
+    }
+    private void moveInToOut(){
+        if(out.isEmpty()){
+            while(!in.isEmpty()){
+                out.push(in.pop());
+
+            }
+        }
+    }
+
+    public int pop() {
+        moveInToOut();
+        return out.pop();
+
+    }
+
+    public int peek() {
+        moveInToOut();
+        return out.peek();
+
+    }
+
+    public boolean empty() {
+        return in.isEmpty() && out.isEmpty();
+
+    }
+}
+
+/**
+ * Your MyQueue object will be instantiated and called as such:
+ * MyQueue obj = new MyQueue();
+ * obj.push(x);
+ * int param_2 = obj.pop();
+ * int param_3 = obj.peek();
+ * boolean param_4 = obj.empty();
+ */

Problem2.java

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+// Time Complexity : 
+// put() = O(1) average, O(n) worst
+// get() = O(1) average, O(n) worst
+// remove() = O(1) average, O(n) worst
+//
+// Space Complexity : O(N) where N = number of inserted key-value pairs
+//
+// Did this code successfully run on Leetcode : Yes
+//
+// Any problem you faced while coding this : 
+// Understanding how to implement separate chaining using linked lists
+// and handling collisions correctly. Also ensuring find() returns
+// the previous node to simplify insert/update/remove operations.
+
+// Your code here along with comments explaining your approach
+
+class MyHashMap {
+
+    // Node structure for each key-value pair
+    class Node {
+        int key, value;
+        Node next;
+
+        Node(int key, int value) {
+            this.key = key;
+            this.value = value;
+        }
+    }
+
+    // Bucket size (large enough to minimize collisions)
+    private final int SIZE = 10000;
+    private Node[] buckets;
+
+    public MyHashMap() {
+        buckets = new Node[SIZE];
+    }
+
+    // Hash function
+    private int hash(int key) {
+        return key % SIZE;
+    }
+
+    // Helper function: returns previous node of the target key
+    private Node find(Node head, int key) {
+        Node prev = null;
+        Node curr = head;
+
+        while (curr != null && curr.key != key) {
+            prev = curr;
+            curr = curr.next;
+        }
+
+        return prev;
+    }
+
+    public void put(int key, int value) {
+        int index = hash(key);
+
+        // If bucket empty, create a dummy node
+        if (buckets[index] == null) {
+            buckets[index] = new Node(-1, -1);
+        }
+
+        Node prev = find(buckets[index], key);
+
+        // If the key does NOT exist, add new node
+        if (prev.next == null) {
+            prev.next = new Node(key, value);
+        } else {
+            // If key exists → update value
+            prev.next.value = value;
+        }
+    }
+
+    public int get(int key) {
+        int index = hash(key);
+
+        if (buckets[index] == null) return -1;
+
+        Node prev = find(buckets[index], key);
+
+        if (prev.next == null) return -1;
+
+        return prev.next.value;
+    }
+
+    public void remove(int key) {
+        int index = hash(key);
+
+        if (buckets[index] == null) return;
+
+        Node prev = find(buckets[index], key);
+
+        // If key exists, delete it
+        if (prev.next != null) {
+            prev.next = prev.next.next;
+        }
+    }
+}
+
+/**
+ * Your MyHashMap object will be instantiated and called as such:
+ * MyHashMap obj = new MyHashMap();
+ * obj.put(key,value);
+ * int param_2 = obj.get(key);
+ * obj.remove(key);
+ */