LeetCode – Min Stack – 30Days Challenge

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

1. push(x) — Push element x onto stack.
2. pop() — Removes the element on top of the stack.
3. top() — Get the top element.
4. getMin() — Retrieve the minimum element in the stack.

Example 1:

Input [“MinStack”,”push”,”push”,”push”,”getMin”,”pop”,”top”,”getMin”] [[],[-2],[0],[-3],[],[],[],[]] Output [null,null,null,null,-3,null,0,-2] Explanation MinStack minStack = new MinStack(); minStack.push(-2); minStack.push(0); minStack.push(-3); minStack.getMin(); // return -3 minStack.pop(); minStack.top(); // return 0 minStack.getMin(); // return -2

Constraints:

Methods pop, top and getMin operations will always be called on non-empty stacks.

Solution

To get min element in constant time, we need two collections to store one for the actual data and other for min value.

Code


import java.util.ArrayList;
import java.util.List;

class MinStack {

    /** initialize your data structure here. */
    List<Integer> list;
    List<Integer> min;
    public MinStack() {
        list=new ArrayList<Integer>();
        min=new ArrayList<Integer>();
    }
    
    public void push(int x) {
        list.add(x);
     
        if(min.size()==0) min.add(x);
        else {
            min.add(Math.min(min.get(min.size()-1),x));
        }
    }
    
    public void pop() {
        min.remove(min.size()-1);
        list.remove(list.size()-1);
    }
    
    public int top() {
        return list.get(list.size()-1);
    }
    
    public int getMin() {
        return min.get(min.size()-1);
    }
}

public class MinStackTest {

	public static void main(String[] args) {
		MinStack minStack = new MinStack();
		minStack.push(-2);
		minStack.push(0);
		minStack.push(-3);
		System.out.println(minStack.getMin()); 
		minStack.pop();
		System.out.println(minStack.top());    
		System.out.println(minStack.getMin());
	}

}