155 Min Stack 最小栈

设计一个支持 push，pop，top 操作，并能在常量时间内检索最小元素的栈。
    push(x) -- 将元素x推入栈中。
    pop() -- 删除栈顶的元素。
    top() -- 获取栈顶元素。
    getMin() -- 检索栈中的最小元素。
示例:
MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin();   --> 返回 -3.
minStack.pop();
minStack.top();      --> 返回 0.
minStack.getMin();   --> 返回 -2.
详见：https://leetcode.com/problems/min-stack/description/

Java实现：

方法一：

class MinStack {
    private Stack<Integer> stk;
    private Stack<Integer> minStk;

    /** initialize your data structure here. */
    public MinStack() {
        stk=new Stack<Integer>();
        minStk=new Stack<Integer>();
    }
    
    public void push(int x) {
        stk.push(x);
        if(minStk.isEmpty()){
            minStk.push(x);
        }else if(minStk.peek()>=x){
            minStk.push(x);
        }
    }
    
    public void pop() {
        int top=stk.pop();
        if(minStk.peek()==top){
            minStk.pop();
        }
    }
    
    public int top() {
        return stk.peek();
    }
    
    public int getMin() {
        return minStk.peek();
    }
}

/**
 * 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();
 */

方法二：

class MinStack {
    private Stack<Integer> stk;
    private Stack<Integer> minStk;

    /** initialize your data structure here. */
    public MinStack() {
        stk=new Stack<Integer>();
        minStk=new Stack<Integer>();
    }
    
    public void push(int x) {
        stk.push(x);
        if(minStk.isEmpty()){
            minStk.push(x);
        }else if(minStk.peek()>=x){
            minStk.push(x);
        }else{
            minStk.push(minStk.peek());
        }
    }
    
    public void pop() {
        stk.pop();
        minStk.pop();
    }
    
    public int top() {
        return stk.peek();
    }
    
    public int getMin() {
        return minStk.peek();
    }
}

/**
 * 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();
 */