解题思路：

辅助栈

class MinStack {private Stack<Integer> stack;private Stack<Integer> min_stack;public MinStack() {stack = new Stack<>();min_stack = new Stack<>();}public void push(int val) {stack.push(val);if (min_stack.isEmpty() || val <= min_stack.peek())min_stack.push(val);}public void pop() {if (stack.pop().equals(min_stack.peek()))min_stack.pop();}public int top() {return stack.peek();}public int getMin() {return min_stack.peek();}
}