最小值双栈

准备两个栈构成新的栈结构

• 一个栈把所有的元素都压进去，作为一个普通栈（数据栈）
• 另外一个栈每次压入最小的数（最小值栈）：每有一个数进栈时，都跟这个最小值栈的栈顶比较大小，如果更小，则把新的数压入两个栈中，如果大于栈顶的数，则把新的数压入数据栈，最小值栈重复压入当前最小值
• 这样的话，两个栈的元素都是一样多的，可以做到三个操作都是O(1)
• 每次取数，都在各自的栈里边取数
  

import java.util.Stack;

public class GetMinStack {

public static class MyStack1 {

	private Stack<Integer> stackData;
	private Stack<Integer> stackMin;

	//结构1
	public MyStack1() {
		this.stackData = new Stack<Integer>();
		this.stackMin = new Stack<Integer>();
	}
	
	//进栈
	public void push(int newNum) {
		if (this.stackMin.isEmpty()) {
			this.stackMin.push(newNum);
		} else if (newNum <= this.getmin()) {
			this.stackMin.push(newNum);
		}
		this.stackData.push(newNum);
	}

	//出栈
	public int pop() {
		if (this.stackData.isEmpty()) {
			throw new RuntimeException("Your stack is empty.");
		}
		int value = this.stackData.pop();
		if (value == this.getmin()) {
			this.stackMin.pop();
		}
		return value;
	}

	//获取最小值
	public int getmin() {
		if (this.stackMin.isEmpty()) {
			throw new RuntimeException("Your stack is empty.");
		}
		return this.stackMin.peek();
	}
}

	//结构2
	public static class MyStack2 {
		private Stack<Integer> stackData;
		private Stack<Integer> stackMin;

	public MyStack2() {
		this.stackData = new Stack<Integer>();
		this.stackMin = new Stack<Integer>();
	}

	//进栈
	public void push(int newNum) {
		if (this.stackMin.isEmpty()) {
			this.stackMin.push(newNum);
		} else if (newNum < this.getmin()) {
			this.stackMin.push(newNum);
		} else {
			int newMin = this.stackMin.peek();
			this.stackMin.push(newMin);
		}
		this.stackData.push(newNum);
	}

	//出栈
	public int pop() {
		if (this.stackData.isEmpty()) {
			throw new RuntimeException("Your stack is empty.");
		}
		this.stackMin.pop();
		return this.stackData.pop();
	}

	//获取最小值
	public int getmin() {
		if (this.stackMin.isEmpty()) {
			throw new RuntimeException("Your stack is empty.");
		}
		return this.stackMin.peek();
	}
}

	public static void main(String[] args) {
		MyStack1 stack1 = new MyStack1();
		stack1.push(3);
		System.out.println(stack1.getmin());
		stack1.push(4);
		System.out.println(stack1.getmin());
		stack1.push(1);
		System.out.println(stack1.getmin());
		System.out.println(stack1.pop());
		System.out.println(stack1.getmin());

		System.out.println("=============");

		MyStack1 stack2 = new MyStack1();
		stack2.push(3);
		System.out.println(stack2.getmin());
		stack2.push(4);
		System.out.println(stack2.getmin());
		stack2.push(1);
		System.out.println(stack2.getmin());
		System.out.println(stack2.pop());
		System.out.println(stack2.getmin());
	}
}

`