MinStack1

gistfile1.txt

import java.util.Stack;

/*
 * 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.
 */

/**
 * Space     - O(2n) 
 * push(x)   - O(1)
 * pop()     - O(1)
 * top()     - O(1)
 * getMin()  - O(1)
 * @author hongfeili
 *
 */
public class MinStack {

    private Stack<Integer> s;
    private Stack<Integer> min;

    public MinStack() {
        s = new Stack();
        min = new Stack();
    }

    public void push(int x) {
        s.push(x);
        if (min.size() != 0) {
            min.push(min.peek() < x ? min.peek() : x);
        } else {
            min.push(x);
        }
    }

    public void pop() {
        min.pop();
        s.pop();
    }

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

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

}