155. Min Stack.java

class MinStack {
    Stack<Integer> mainStack;
    Stack<Integer> minStack;
    public MinStack() {
        mainStack = new Stack<Integer>();
        minStack = new Stack<Integer>();
    }
    
    public void push(int val) {
        mainStack.push(val);
        // if minStack is empty or val (currentVal) < minStack.peek() value
        // so push in minStack

        if(minStack.isEmpty() || val <= minStack.peek()){
            minStack.push(val);
        }else{
            minStack.push(minStack.peek());
        }
    }
    
    public void pop() {
        mainStack.pop();
        minStack.pop();
    }
    
    public int top() {
        return mainStack.peek();
    }
    
    public int getMin() {
       return minStack.peek();
    }
}

/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack obj = new MinStack();
 * obj.push(val);
 * obj.pop();
 * int param_3 = obj.top();
 * int param_4 = obj.getMin();
 */