both solutions added

HashSet.java

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+/*
+The average time complexity of add, put and remove is O(1).
+
+Here, we use both arrays and linkedlist to solve the problem. First the index of the key is found. Then we use traverse through the linked
+list if any to perform the operations.
+
+Yes, the solution was run of leetcode.
+ */
+class MyHashSet {
+    Node arr[];
+    int size;
+    /** Initialize your data structure here. */
+    public MyHashSet() {
+        arr = new Node[100000];
+        size = 100000;
+    }
+
+    public void add(int key) {
+        int index = key%size;
+        Node root = arr[index];
+        if(root==null){
+            arr[index] = new Node(key);
+        }
+        else{
+            Node prev = null;
+            while(root!=null){
+                if(root.key == key){return;}
+                prev = root;
+                root = root.next;
+            }
+            prev.next = new Node(key);
+        }
+    }
+
+    public void remove(int key) {
+        int index = key%size;
+        Node root = arr[index];
+        if(root==null){
+            return;
+        }
+        else{
+            Node prev = null;
+            while(root!=null){
+                if(root.key==key){
+                    if(prev==null){
+                        arr[index] = root.next;
+                        return;
+                    }
+                    else{
+                        prev.next = root.next;
+                        return;
+                    }
+                }
+                prev = root;root = root.next;
+            }
+        }
+    }
+
+    /** Returns true if this set contains the specified element */
+    public boolean contains(int key) {
+        int index = key%size;
+        Node root = arr[index];
+        if(root==null){
+            return false;
+        }
+        else{
+            while(root!=null){
+                if(root.key==key){
+                    return true;
+                }
+                root = root.next;
+            }
+
+            return false;
+        }
+    }
+}
+
+class Node{
+    int key;
+    Node next;
+
+    Node(int key){
+        this.key = key;
+    }
+}
+
+/**
+ * Your MyHashSet object will be instantiated and called as such:
+ * MyHashSet obj = new MyHashSet();
+ * obj.add(key);
+ * obj.remove(key);
+ * boolean param_3 = obj.contains(key);
+ */

QueueUsingStack.java

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+/*
+The amortized time complexity is O(1) and the space complexity is O(N) where N is the number of elements stored.
+
+Here, the idea is to use two stacks to implement queue. One for insertion and the other for pop of peek. When we want to peek or pop and
+the second stack is empty we pop each element from first stack and insert into the second stack.
+
+Yes, the solution passed all the test cases in leetcode
+ */
+class MyQueue {
+
+    Stack<Integer> stack;
+    Stack<Integer> queue;
+    /** Initialize your data structure here. */
+    public MyQueue() {
+        stack = new Stack<>();
+        queue = new Stack<>();
+    }
+
+    /** Push element x to the back of queue. */
+    public void push(int x) {
+        stack.push(x);
+    }
+
+    /** Removes the element from in front of queue and returns that element. */
+    public int pop() {
+        if(queue.isEmpty()){
+            while(stack.size()>0){
+                queue.push(stack.pop());
+            }
+        }
+        return queue.pop();
+    }
+
+    /** Get the front element. */
+    public int peek() {
+        if(queue.isEmpty()){
+            while(stack.size()>0){
+                queue.push(stack.pop());
+            }
+        }
+        return queue.peek();
+    }
+
+    /** Returns whether the queue is empty. */
+    public boolean empty() {
+        return queue.isEmpty()&&stack.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();
+ */