155. Min Stack

Posted 2022-12-05 Carl_Hugo

https://leetcode.com/problems/min-stack/description/

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.

解题思路：
创建一个栈，和变量min，每push一个元素，判断是否是最小的元素。

public class MinStack 

    int min=Integer.MAX_VALUE;
    Stack<Integer> stack; 

    /** initialize your data structure here. */
    public MinStack() 
        stack = new Stack<>();
    

    public void push(int x) 
        if(x<=min)
            //如果x是最小值，就将
            stack.push(min);
            min=x;
        
        stack.push(x);
    

    public void pop() 
        if(min==stack.pop())min = stack.pop();
    

    public int top() 
        return stack.peek();
    

    public int getMin() 
        return min;
    


/**
 * 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();
 */