LeetCode : 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.

class MinStack {
private:
    stack<int> s1;
    stack<int> s2;
public:
    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();
    }
};