_155.java

package com.fishercoder.solutions;

import java.util.*;

/**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.
 Example:
 _155 minStack = new _155();
 minStack.push(-2);
 minStack.push(0);
 minStack.push(-3);
 minStack.getMin();   --> Returns -3.
 minStack.pop();
 minStack.top();      --> Returns 0.
 minStack.getMin();   --> Returns -2.
 */
public class _155 {
    
    private Stack<Integer> stack;
    private int min;

    /** initialize your data structure here. */
    public _155() {
        stack = new Stack();
        min = Integer.MAX_VALUE;
    }
    
    public void push(int x) {
        /**All the trick happens here, we push the second minimum number onto the stack before we push the newer one,
         * this way, when popping, we could always get the next minimum one in constant time.*/
        if(x <= min){
            stack.push(min);
            min = x;
        }
        stack.push(x);
    }
    
    public void pop() {
        if(min == stack.peek()){
            stack.pop();
            min = stack.pop();
        } else {
            stack.pop();
        }
        if(stack.isEmpty()) min = Integer.MAX_VALUE;
    }
    
    public int top() {
        return stack.peek();
    }
    
    public int getMin() {
        return min;
    }

}