LeetCode 155. Min Stack（最小栈）

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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<Integer> stack = new ArrayList<>();    private List<Integer> 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);    }}

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