leetcode Min Stack 最小栈

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.

  题意：设计一个栈支持push、pop、top和返回最小值的操作
  思路：用两个栈实现，其中一个栈维持每次push和pop之后的最小值。

class MinStack {private:    stack<int> s1;    stack<int> s2;public:    /** initialize your data structure here. */       void push(int x) {        s1.push(x);        if (s2.empty() || x <= getMin()) s2.push(x);    }    void pop() {        if (s1.top() == getMin()) s2.pop();        s1.pop();    }    int top() {        return s1.top();    }    int getMin() {        return s2.top();    }};/** * 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(); */