155. Min Stack

QUESTION:

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.

EXPLANATION:

可以直接使用list集合就可以实现，同时我也在leetcode上发现了更加简洁的实现，但是这个实现的话是将原本的数据与min的数据结合在一起，确实可以起到符合题目的目的，但是如果求到size之类的情况时候，原始的数据其实已经被污染了，就没有办法找到一共push了多少个数字进去。

而且如今的电脑性能也许真的不用在意这个了。所以我觉得还是list的解决办法更为靠谱一点。

SOLUTION:

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


public class MinStack {

    ArrayList<Integer> list;
        Stack<Integer> instance;
        /** initialize your data structure here. */
        public MinStack() {
            instance = new Stack<>();
            list = new ArrayList<>();
        }

        public void push(int x) {
            instance.push(x);
            list.add(x);
            Collections.sort(list);
        }

        public void pop() {
            int tmp = instance.pop();
            list.remove((Object)tmp);
        }

        public int top() {
            return instance.peek();
        }

        public int getMin() {
            return list.get(0);
        }
}
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--> return <--