Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.
Implement the MinStack class:
MinStack() initializes the stack object.
void push(int val) pushes the element val onto the stack.
void pop() removes the element on the top of the stack.
int top() gets the top element of the stack.
int getMin() retrieves the minimum element in the stack.
class MinStack {
// 数据栈
private Stack<Integer> data;
// 辅助栈
private Stack<Integer> helper;
public MinStack() {
data = new Stack<>();
helper = new Stack<>();
}
// 思路 1：数据栈和辅助栈在任何时候都同步
public void push(int x) {
// 数据栈和辅助栈一定会增加元素
data.add(x);
if (helper.isEmpty() || helper.peek() >= x) {
helper.add(x);
} else {
helper.add(helper.peek());
}
}
public void pop() {
// 两个栈都得 pop
if (!data.isEmpty()) {
helper.pop();
data.pop();
}
}
public int top() {
if(!data.isEmpty()){
return data.peek();
}
throw new RuntimeException("栈中元素为空，此操作非法");
}
public int getMin() {
if(!helper.isEmpty()){
return helper.peek();
}
throw new RuntimeException("栈中元素为空，此操作非法");
}
}
/**
* 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();
*/
