155. Min 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.

 

// constructor 后面要有括号
// java中取最大的那个数字是peek(偷看一次不拿出来）不是peak（顶峰，这个用pop代替了）
// minStack.isEmpty() 的E是大写

 

 

class MinStack {
    private Stack minStack;
    private Stack Stack;
    
    public MinStack(){
        minStack = new Stack();
        Stack = new Stack();       
    }
    
    public void push(int x) {
        Stack.push(x);
        if(minStack.isEmpty()){
            minStack.push(x);
        }else{
            minStack.push(Math.min(x, minStack.peek()));
        }
    }

    public void pop() {
        minStack.pop();
        Stack.pop();
    }

    public int top() {
        minStack.peek();
        return Stack.peek();
    }

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