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.

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.

思路：在每一个最小值下面放置一个未加入前的最小值（全局次小值）

class MinStack {

    /** initialize your data structure here. */
    List<Integer> stack;
    int min;
    /** initialize your data structure here. */
    public MinStack() {
        stack=new ArrayList<Integer>();
        min=Integer.MAX_VALUE;
    }
    
    public void push(int x) {
        if(x<=min)
        {
            stack.add(min);
            min=x;
        }
        stack.add(x);
    }
    
    public void pop() {
        if(stack!=null&&stack.size()!=0)
        {
            int top=stack.remove(stack.size()-1);
            if(top==min)
            {
                min=stack.remove(stack.size()-1);
            }
        }
    }
    
    public int top() {
        if(stack==null||stack.size()==0)
        {
            return -1;
        }
        else
        {
            return stack.get(stack.size()-1);
        }
    }
    
    public int getMin() {
        return min;
    }
}