栈&最小栈

栈是后入先出（LIFO）的数据结构，首先处理添加到队列的最新元素。

插入操作称作入栈push，在堆栈的末尾添加一个新元素。删除操作称作pop，删除最后一个元素。

动态数组即可实现堆栈。

#include <iostream>

class MyStack {
    private:
        vector<int> data;               // store elements
    public:
        /** Insert an element into the stack. */
        void push(int x) {
            data.push_back(x);
        }
        /** Checks whether the queue is empty or not. */
        bool isEmpty() {
            return data.empty();
        }
        /** Get the top item from the queue. */
        int top() {
            return data.back();
        }
        /** Delete an element from the queue. Return true if the operation is successful. */
        bool pop() {
            if (isEmpty()) {
                return false;
            }
            data.pop_back();
            return true;
        }
};

int main() {
    MyStack s;
    s.push(1);
    s.push(2);
    s.push(3);
    for (int i = 0; i < 4; ++i) {
        if (!s.isEmpty()) {
            cout << s.top() << endl;
        }
        cout << (s.pop() ? "true" : "false") << endl;
    }
}

一些基本操作的代码示例：

#include <iostream>

int main() {
    // 1. Initialize a stack.
    stack<int> s;
    // 2. Push new element.
    s.push(5);
    s.push(13);
    s.push(8);
    s.push(6);
    // 3. Check if stack is empty.
    if (s.empty()) {
        cout << "Stack is empty!" << endl;
        return 0;
    }
    // 4. Pop an element.
    s.pop();
    // 5. Get the top element.
    cout << "The top element is: " << s.top() << endl;
    // 6. Get the size of the stack.
    cout << "The size is: " << s.size() << endl;
}

最小栈：检索到最小元素的栈

设计支持push,pop,top操作，在常数时间内检索到最小元素的栈。实现MinStack类。

class MinStack {
private:
    stack<long> st;
    long min;
public:
    //初始化堆栈对象
    MinStack() { 
    }
    
    /*将元素val推入堆栈。
    如推入元素依次为1,2,6,4 -> 0,1,5,3. 3,2,4,1 -> 0,-1,2,-1.
    */
    void push(int val) {
        if(!st.empty()){
            long long diff=val-min; 
            min=diff>0 ? min:val;   //栈非空时存储与当前min的差值
            st.push(diff);
        }
        else{
            min=val;    //栈为空时将val赋值给min
            st.push(0); //栈的第一个元素为0
        }
    }
    
    void pop() {
        if(!st.empty()){
            long long diff=st.top();
            st.pop();
            if(diff<0) min-=diff; //diff为负时说明top()比当前min小，所以要更新min
        }
    }
    
    int top() {
        return st.top()<0? min:(min+st.top());
    }
    
    int getMin() {
        return min;
    }
};

/**
 * 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();
 */

顺利运行通过，但是我没有懂得为什么这里需要定义long类型。将long和long long都改为int类型后发现示例运行结果正确，但是提交后报错： 

Line 11: Char 25: runtime error: signed integer overflow: -2147483648 - 2147483647 cannot be represented in type 'int' (solution.cpp) SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior prog_joined.cpp:16:25