LeetCode_155. Min Stack

155. Min Stack

Easy

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:

MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin();   --> Returns -3.
minStack.pop();
minStack.top();      --> Returns 0.
minStack.getMin();   --> Returns -2.

package leetcode.easy;

import java.util.Stack;

public class MinStack {
	private Stack<Integer> stack = new Stack<Integer>();
	private Stack<Integer> min_stack = new Stack<Integer>();

	/** initialize your data structure here. */
	public MinStack() {

	}

	public void push(int x) {
		stack.push(x);
		if (min_stack.isEmpty() || (!min_stack.isEmpty() && x <= min_stack.peek())) {
			min_stack.push(x);
		}
	}

	public void pop() {
		if (!stack.isEmpty()) {
			if (stack.peek().equals(min_stack.peek())) {
				min_stack.pop();
			}
			stack.pop();
		}
	}

	public int top() {
		if (!stack.isEmpty()) {
			return stack.peek();
		}
		return Integer.MIN_VALUE;
	}

	public int getMin() {
		if (!min_stack.isEmpty()) {
			return min_stack.peek();
		}
		return Integer.MIN_VALUE;
	}

	@org.junit.Test
	public void test() {
		MinStack minStack = new MinStack();
		minStack.push(-2);
		minStack.push(0);
		minStack.push(-3);
		System.out.println(minStack.getMin());// --> Returns -3.
		minStack.pop();
		System.out.println(minStack.top());// --> Returns 0.
		System.out.println(minStack.getMin());// --> Returns -2.
	}
}

/**
 * Your MinStack object will be instantiated and called as such: MinStack obj =
 * new MinStack(); obj.push(x); obj.pop(); int param_3 = obj.top(); int param_4
 * = obj.getMin();
 */