232. 用栈实现队列

class MyQueue
{
private:stack<int> in_stack, out_stack;void in2out(){while (!in_stack.empty()){out_stack.push(in_stack.top());in_stack.pop();}}
public:MyQueue(){}void push(int x){in_stack.push(x);}int pop(){if (out_stack.empty()){in2out();}auto temp{ out_stack.top() };out_stack.pop();return temp;}int peek(){if (out_stack.empty()){in2out();}return out_stack.top();}bool empty(){return in_stack.empty() && out_stack.empty();}
};

225. 用队列实现栈

class MyStack
{
private:queue<int> stack;
public:MyStack(){}void push(int x){stack.push(x);if (stack.size() > 1){for (int i = 0; i < stack.size() - 1; i++){auto temp{ stack.front() };stack.pop();stack.push(temp);}}}int pop(){auto temp{ stack.front() };stack.pop();return temp;}int top(){return stack.front();}bool empty(){return stack.empty();}
};

20. 有效的括号

class Solution
{
public:bool isValid(string s){stack<char> stk;for (auto& ch : s){if (ch == '(' || ch == '{' || ch == '['){stk.push(ch);}else if (stk.empty()){return false;}else{if (stk.top() == '(' && ch == ')' ||stk.top() == '{' && ch == '}' ||stk.top() == '[' && ch == ']'){stk.pop();}else{return false;}}}return stk.empty();}
};

1047. 删除字符串中的所有相邻重复项 

class Solution
{
public:string removeDuplicates(string s){string ans;for (auto& ch : s){if (ans.empty()){ans.push_back(ch);}else{if (ans.back() == ch){ans.pop_back();}else{ans.push_back(ch);}}}return ans;}
};