Min Stack

Description：

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.

Code：

class MinStack {
public:
    void push(int x) {
        m_stack.push(x);
        if (min_num.empty() || x<min_num.top())
           min_num.push(x);
        else
           min_num.push(min_num.top());
    }

    void pop() {
        assert( !m_stack.empty() && !min_num.empty() );
        m_stack.pop();
        min_num.pop();
    }

    int top() {
            assert( !m_stack.empty() );
            return m_stack.top();
    }

    int getMin() {
        return min_num.top();
    }
private:
    stack<int>m_stack;
    stack<int>min_num;
};