LeetCode – Min Stack (Java)

Category: Algorithms   February 21, 2014

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

push(x) -- Push element x onto stack.
pop() -- Removes the element on top of the stack.
top() -- Get the top element.
getMin() -- Retrieve the minimum element in the stack.

Java Solution

To make constant time of getMin(), we need to keep track of the minimum element for each element in the stack. Define an element class that holds element value, min value, and pointer to elements below it.

class Elem{
    public int value;
    public int min;
    public Elem next;
 
    public Elem(int value, int min){
        this.value = value;
        this.min = min;
    }
}
 
public class MinStack {
    public Elem top;
 
    /** initialize your data structure here. */
    public MinStack() {
 
    }
 
    public void push(int x) {
        if(top == null){
            top = new Elem(x, x);
        }else{
            Elem e = new Elem(x, Math.min(x,top.min));
            e.next = top;
            top = e;
        }
 
    }
 
    public void pop() {
        if(top == null)
            return;
        Elem temp = top.next;
        top.next = null;
        top = temp;
 
    }
 
    public int top() {
        if(top == null)
            return -1;
        return top.value;
    }
 
    public int getMin() {
        if(top == null)
            return -1;
        return top.min;
    }
}

Related posts:

  1. LeetCode – Basic Calculator (Java)
  2. LeetCode – Implement Queue using Stacks (Java)
  3. LeetCode – Implement Stack using Queues (Java)
  4. Implement a Stack Using an Array in Java
Category >> Algorithms  
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  • Alik Elzin

    Same solution but with encapsulation and null (instead of -1)


    /* TODO: support comparable or use comparator.*/
    public class MinStack
    {
    private Node top;

    public Integer top()
    {
    if (top == null) return null;
    return top.value;
    }

    public void push(int x)
    {
    if (top == null)
    {
    top = new Node(x, x, null);
    return;
    }

    Node n = new Node(x, top.min, top);
    top = n;
    }

    public Integer pop()
    {
    if (top == null) return null;

    int x = top.value;
    top = top.prev;

    return x;
    }

    public Integer getMin()
    {
    if (top == null) return null;

    return top.min;
    }

    private static class Node
    {
    public final int value;
    public final int min;
    public final Node prev;

    public Node(int value, int min, Node prev)
    {
    this.value = value;
    this.min = min;
    this.prev = prev;
    }
    }
    }

  • Bhaskar

    One would also need a magic wand to get this code work.

  • Yes that’s right, if we use PriorityQueue, push() and pop() won’t be constant time O(1), it will be logarithmic O(logn), where n is number of elements in the stack.

  • Marina


    package base_alg;

    public class MinStack {

    private Node head;

    public int getMin() {
    return head.getMin();
    }
    public void push(int value) {
    if (head == null) {
    head = new Node(value);
    } else {

    head = new Node(value, head, Math.min(value, head.getMin()));
    }
    }

    public int top() {
    if (head == null) {
    throw new NullPointerException();
    }

    return head.getValue();
    }

    public int pop() {
    int value = top();
    head = head.getNext();
    return value;
    }

    }

    class Node {
    private int value;
    private int min;
    private Node next;

    public Node(int value) {
    this.value = value;
    this.min = value;
    }

    public Node(int value, Node node, int min) {
    this.next = node;
    this.value = value;
    this.min = min;
    }

    public void setNext(Node next) {
    this.next = next;
    }

    public void setValue(int value) {
    this.value = value;
    }

    public Node getNext() {
    return next;
    }

    public int getValue() {
    return value;
    }

    public int getMin() {
    return min;
    }

    public void setMin(int min) {
    this.min = min;
    }
    }

  • Krishnan Rajagopalan

    Will this code work? Suppose that when a new element is inserted, it is known that the min(x, top.min) = x.

    In that case, the ‘min’ value for all the remaining elements of the stack are not updated. So when you pop an element from the stack you need to update the ‘min’ value for the new top with the ‘min’ value f the current top that is being removed.

  • Asad Arhaan

    Using min heap.

    class MinStack {
    public:
    stack st;
    priority_queue<int,vector,greater> pq;

    void push(int x){
    st.push(x);
    pq.push(x);
    }

    void pop(){
    if(st.top()==pq.top()){
    st.pop();
    pq.pop();
    }
    else{
    st.pop();
    }
    }

    int top(){
    return st.top();
    }

    int getMin(){
    return pq.top();
    }

    };

  • Justin Heyes-Jones

    All the operations in the above example are constant time. Where is the iteration you’re seeing? If you don’t follow how the min is tracked in the stack values try working through the algorithm on paper.

  • Justin Heyes-Jones

    Using a priority queue gives you a program that is correct but does not meet the constant time performance requirements. Whilst peek can be constant time both push and pop will be logarithmic.

  • Gayathri Muralidharan

    public class MinStack {
    PriorityQueue pq;
    Stack st;
    /** initialize your data structure here. */
    public MinStack() {
    pq=new PriorityQueue();
    st=new Stack();
    }

    public void push(int x) {
    pq.offer(x);
    st.push(x);
    }

    public void pop() {
    Integer x = st.pop();
    pq.remove(x);
    }

    public int top() {
    return st.peek();
    }

    public int getMin() {
    return pq.peek();
    }
    }

  • mukthar ahmed

    I felt the code is all good, delivering to what is required. At a given point of time,/node leve, we need the min-value in the stack and every node carrying a property (Node.min) is satisfied O(1)

  • Here is my code. I guess it works fine !

    class MinStack {
    Node head;

    public void push(int x) {
    if(head == null) {
    head = new Node(x);
    head.min = x;
    }
    else {
    Node temp = new Node(x);
    if(temp.min > head.min) temp.min = head.min;
    temp.next = head;
    head = temp;
    }
    }

    public void pop() {
    if(head == null ) return ;
    head = head.next;
    }

    public int top() {
    if(head == null ) return Integer.MAX_VALUE;
    return head.value;
    }

    public int getMin() {
    if(head == null ) return Integer.MAX_VALUE;
    return head.min;
    }
    }

    class Node {
    int value;
    int min;
    Node next;
    Node(int x) { value = x; next = null; min = x;}
    }

  • Tank

    Actually, this works:

    class MinStack {

    int[] arr = new int [100];

    int index = -1;

    int[] min = new int [100];

    public void push(int x) {

    if (index == arr.length – 1) {

    arr = Arrays.copyOf(arr, 2*arr.length);

    min = Arrays.copyOf(min, 2*min.length);

    }

    index++;

    arr[index] = x;

    min[index] = index == 0 ? x : Math.min(x, min[index-1]);

    }

    public void pop() {

    if(index == arr.length/2 – 1 && arr.length > 100) {

    arr = Arrays.copyOf(arr, arr.length/2);

    min = Arrays.copyOf(min, min.length/2);

    }

    index–;

    }

    public int top() {

    if (index > -1) {

    return arr[index];

    }

    else {

    return -1;

    }

    }

    public int getMin() {

    if(index > -1) {

    return min[index];

    }

    else {

    return -1;

    }

    }

    }

  • Tank

    I guess you could add it to a HashMap, along with the number of times that min value appears.. so if the min value gets popped, you either remove it from the hash map if its value is 1, or decrement the # of times it appears

  • MyCat

    Just keep the min value is not enough, if your next pop removes the min value, then you need know the new min value, which still needs O(n) time to search

  • Tank

    you’re right. You should have a minimum value declared in this class, and every time you push a new number to the stack, check if it is a lower value than the current min, and update that way, so when you call getMin it’s 0(1)

  • Justin Stein

    Am I having a brain fart? How is this retrieving the minimum value in constant runtime if it iterates through the entire stack? That is O(n) runtime.