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.

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();
}
}