[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.

分析：用两个栈来实现

class MinStack {
    // normal stack
    Stack<Integer> st = new Stack<Integer>();
    // min value stack
    Stack<Integer> stm = new Stack<Integer>();
    
    public void push(int x) {
        st.push(x);
        if (stm.isEmpty() || stm.peek() >= x)
            stm.push(x);
    }

    public void pop() {
        int peek = st.peek();
        st.pop();
        if (peek <= stm.peek())
            stm.pop();
    }

    public int top() {
        return st.peek();
    }

    public int getMin() {
        return stm.peek();
    }
}