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.

双stack

public class MinStack {

   // stack: store the stack numbers  
    private Stack<Integer> stack = new Stack<Integer>();  
    // minStack: store the current min values  
    private Stack<Integer> minStack = new Stack<Integer>();  

    public void push(int x) {  
        // store current min value into minStack  
        if (minStack.isEmpty() || x <= minStack.peek())  
            minStack.push(x);  
        stack.push(x);  
    }  

    public void pop() {  
        // use equals to compare the value of two object, if equal, pop both of them  
        if (stack.peek().equals(minStack.peek()))  
            minStack.pop();  
        stack.pop();  
    }  

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

    public int getMin() {  
        return minStack.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();
 */

两个stack，优化第二个空间，重复的最小值不push

public class MinStack {
    private Stack<Integer> stack;
    private Stack<Integer> minStack;
    public MinStack() {
        // do initialize if necessary
        stack = new Stack<Integer>();
        minStack = new Stack<Integer>();
    }

    public void push(int number) {
        // write your code here
        stack.push(number);
        if( minStack.isEmpty()){
            minStack.push(number);
        }else if( number <= minStack.peek()){
            minStack.push(number);
        }
    }

    public int pop() {
        // write your code here
        int p = stack.pop();
        if( p == minStack.peek())
            minStack.pop();
        return p;
    }

    public int min() {
        // write your code here
        return minStack.peek();
    }
}

一个stack

public class MinStack {
    private Stack<Integer> stack = new Stack<>();
    private int min = Integer.MAX_VALUE;


    public void push(int x) {
        if (x <= min) {
            stack.push(min);
            min = x;
        }
        stack.push(x);
    }

    public void pop() {
        int top = stack.pop();
        if (top == min) min = stack.pop();
    }

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

    public int getMin() {
        return min;
    }
}

/**
 * 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();
 */