LeetCode 155. Min Stack（最小栈）

原题网址：https://leetcode.com/problems/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.

方法：在栈中保持多一个信息，当前栈的最小值。

class MinStack {
    private List stack = new ArrayList<>();
    private List mins = new ArrayList<>();
    public void push(int x) {
        stack.add(x);
        if (mins.size() == 0) mins.add(x);
        else mins.add(Math.min(x, mins.get(mins.size()-1)));
    }

    public void pop() {
        int p = stack.get(stack.size()-1);
        stack.remove(stack.size()-1);
        mins.remove(mins.size()-1);
    }

    public int top() {
        return stack.get(stack.size()-1);
    }

    public int getMin() {
        return mins.get(mins.size()-1);
    }
}