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.

分析

维护2个栈，每次都一起增删，长度一直都一样

class MinStack {
    Stack<Integer> s;
    Stack<Integer> min;
    /** initialize your data structure here. */
    public MinStack() {
        s = new Stack<Integer>();
        min = new Stack<Integer>();
    }

    public void push(int x) {
        s.push(x);
        if(min.isEmpty() || min.peek() > x){
            min.push(x);
        }else{
            min.push(min.peek());
        }
    }

    public void pop() {
        s.pop();
        min.pop();
    }

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

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

/**
 * 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();
 */