有效的括号

https://leetcode.cn/problems/valid-parentheses/

class Solution {
public:bool isValid(string s) {stack<char> st;//遍历字符串,碰到左括号：进栈   碰到右括号：出栈顶元素判断是否和该右括号匹配for(auto& ch:s){if(ch == '(' || ch == '[' || ch == '{') {st.push(ch);}else {//如果栈为空,说明括号是不匹配的if(st.empty()) return false;//出栈顶元素和当前右括号匹配char top = st.top();st.pop();if( ch ==')' && top != '('  || ch == ']' && top != '[' ||ch == '}' && top != '{')return false; }}return st.empty(); //如果最后栈为空,说明括号是匹配的}
};

用队列实现栈

https://leetcode-cn.com/problems/implement-stack-using-queues/

两个队列实现栈

class MyStack {
public:MyStack() {}void push(int x) {NonEmptyQueue.push(x);//往不为空的队列插入数据}int pop() {//将不空队列的数据放到空队列当中,直到不空队列只有一个元素while(NonEmptyQueue.size() > 1){EmptyQueue.push(NonEmptyQueue.front());NonEmptyQueue.pop();}int front = NonEmptyQueue.front();NonEmptyQueue.pop();NonEmptyQueue.swap(EmptyQueue);//交换两个队列,保证EmptyQueue为空队列return front;}int top() {return NonEmptyQueue.back();}bool empty() {return EmptyQueue.empty() && NonEmptyQueue.empty();}
private:queue<int> NonEmptyQueue;//不为空的队列queue<int> EmptyQueue;//空队列
};

一个队列实现栈

class MyStack {
public:MyStack() {}void push(int x) {q.push(x);}int pop() {int size = q.size();//将前size-1个元素重新插入到队列当中for(int i = 0;i<size-1;i++){int front = q.front();q.pop();q.push(front);}//此时队头元素就相当于是栈顶元素int front = q.front();q.pop();return front;}int top() {return q.back();}bool empty() {return q.empty();}
private:queue<int> q;
};

用栈实现队列

https://leetcode-cn.com/problems/implement-queue-using-stacks/

class MyQueue {
public:MyQueue() {}void push(int x) {pushStack.push(x);}void Check() //检查是否要将push栈的内容导入到pop栈{if(popStack.empty()){while(!pushStack.empty()){popStack.push(pushStack.top());pushStack.pop();}}}int pop() {Check();int top = popStack.top();popStack.pop();return top;}int peek() {Check();return popStack.top();}bool empty() {return pushStack.empty() && popStack.empty();}
private:stack<int> pushStack;//存放数据的栈stack<int> popStack;//用于弹出数据的栈
};

设计循环队列

https://leetcode-cn.com/problems/design-circular-queue/

方式1：使用数组实现

class MyCircularQueue {
public:MyCircularQueue(int k) {arr.resize(k+1);//开辟k+1个空间front = tail = 0;size = k;}bool enQueue(int value) {  //向循环队列插入一个元素。如果成功插入则返回真if(isFull()) return false;arr[tail] = value;tail ++;tail %= size+1; //tail = tail % (size+1)return true;}bool deQueue() { //从循环队列中删除一个元素。如果成功删除则返回真。if(isEmpty()) return false;front++;front %= size+1;return true;}int Front() {if(isEmpty()) return -1;return arr[front];}int Rear() {if(isEmpty()) return -1;//由于插入元素之后,tail往后走,所以tail的前一个元素才是队尾元素if(tail == 0) return arr[size];else return arr[tail-1];}bool isEmpty() {return front == tail;}bool isFull() {return (tail + 1) % (size+1) == front;}
private:vector<int> arr;int front;//标志队头int tail;//标志队尾int size;//实际存储的元素个数
};

方法2：链表实现

class MyCircularQueue {
public:MyCircularQueue(int k) {//构建k+1个节点的循环链表tail = head = new ListNode;for(int i = 0;i<k;i++) {ListNode* newnode = new ListNode;tail->next = newnode;tail = tail->next;}//tail指向尾节点,让首尾相连tail->next = head;//注意:要让tail回到起始位置！！！！！因为一开始链表没有有效元素head = tail;}bool enQueue(int value) { //向循环队列插入一个元素。如果成功插入则返回真。if(isFull()) return false;tail->val = value;tail = tail->next;return true;}bool deQueue() {if(isEmpty()) return false;head = head->next;return true;}int Front() {if(isEmpty()) return -1;return head->val;}int Rear() {if(isEmpty()) return -1;//从head位置遍历查找,tail的前一个节点放的就算队尾元素ListNode* cur = head;while(cur->next != tail)cur = cur->next;return cur->val;}bool isEmpty() {return head == tail;}bool isFull() {return tail->next == head;}
private:ListNode* head;ListNode* tail;
};s

最小栈

https://leetcode.cn/problems/min-stack/

class MinStack {
public:MinStack() {}void push(int val) {dataStack.push(val);//判断要往辅助栈放重复元素还是更小的元素if( minStack.empty() || minStack.top() >= val)minStack.push(val);else minStack.push(minStack.top());//重复压入当前最小栈栈顶元素}void pop() {int top = dataStack.top();dataStack.pop();minStack.pop(); //坑点！！此时minStack也要同步pop数据}int top() {return dataStack.top();}int getMin() {return minStack.top();}
private:stack<int> minStack;//辅助栈->存放当前栈中对应的最小值stack<int> dataStack;
};

优化：

class MinStack {
public:MinStack() {}void push(int val) {dataStack.push(val);if( minStack.empty() || minStack.top() >= val)minStack.push(val);}void pop() {int top = dataStack.top();dataStack.pop();if(top == minStack.top())minStack.pop(); }int top() {return dataStack.top();}int getMin() {return minStack.top();}
private:stack<int> minStack;//辅助栈->存放当前栈中对应的最小值stack<int> dataStack;
};

栈的压入&弹出序列

https://www.nowcoder.com/practice/d77d11405cc7470d82554cb392585106?tpId=13&&tqId=11174&rp=1&ru=/activity/oj&qru=/ta/coding-interviews/question-ranking