155. 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.

//Java
class MinStack {
    Stack<Integer> stack = new Stack<Integer>();
    Stack<Integer> stack2 = new Stack<Integer>();

    public void push(int x) {
        stack.push(x);
        if(stack2.isEmpty() || x<=getMin())
            stack2.push(x);


    }

    public void pop() {
        int e = stack.pop();
        if(e == getMin())
            stack2.pop();
    }

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

    public int getMin() {
        return stack2.peek();
    }
}

//下面是效率更高的c++代码，思路一样
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();
    }
};