411 stack and queue (20. valid parentheses, 1047. delete all adjacent duplicates in the string, 150. inverse Polish expression evaluation, 239. sliding window maximum, 347. the first k high-frequency elements)

20. Valid parenthesis

class Solution {
    
public:
    bool isValid(string s) {
    

        stack<int> st;
        for (int i = 0; i < s.size(); i++)
        {
    
            if (s[i] == '(')
            {
    
                st.push(')');
            }
            else if (s[i] == '{')
            {
    
                st.push('}');
            }
            else if(s[i] == '[')
            {
    
                st.push(']');
            }
            else if (st.empty() || st.top() != s[i])
            {
    
                return false;
            }
            else
            {
    
                st.pop();
            }
        }

        return st.empty();
    }
};

1047. Delete all adjacent duplicates in the string

class Solution {
    
public:
    string removeDuplicates(string s) {
    
        stack<char> st;

        for (char s1 : s)
        {
    
            if (st.empty() || st.top() != s1)
            {
    
                st.push(s1);
            }
            else
            {
    
                st.pop();
            }
        }

        string result = "";
        while (!st.empty())
        {
    
            result += st.top();
            st.pop();
        }

        reverse(result.begin(), result.end());
        return result;
    }
};

150. Evaluate the inverse Polish expression

class Solution {
    
public:
    int evalRPN(vector<string>& tokens) {
    
        stack<int> st;

        for (int i = 0; i < tokens.size(); i++)
        {
    
            if (tokens[i] == "+" || tokens[i] == "-" || tokens[i] == "*" || tokens[i] == "/")
            {
    
                int num1 = st.top();
                st.pop();
                int num2 = st.top();
                st.pop();

                if (tokens[i] == "+")    st.push(num2 + num1);
                if (tokens[i] == "-")    st.push(num2 - num1);
                if (tokens[i] == "*")    st.push(num2 * num1);
                if (tokens[i] == "/")    st.push(num2 / num1);

            }
            else
            {
    
                st.push(stoi(tokens[i]));
            }
        }
        int res = st.top();
        st.pop();
        return res;
    }
};

239. Maximum sliding window

class Solution {
    
private:
    class MyQueue {
     // A monotonous queue （ From big to small ）
    public:
        deque<int> que; //  Use deque To implement monotonic queues 
        //  Every time it pops up , Compare whether the current value to pop up is equal to the value of the queue exit element , Pop up if equal .
        //  meanwhile pop Judge whether the queue is currently empty .
        void pop(int value) {
    
            if (!que.empty() && value == que.front()) {
    
                que.pop_front();
            }
        }
        //  If push The value of is greater than the value of the entry element , Then pop up the value at the back of the queue , until push The value of is less than or equal to the value of the queue entry element .
        //  This keeps the values in the queue monotonous, from large to small .
        void push(int value) {
    
            while (!que.empty() && value > que.back()) {
    
                que.pop_back();
            }
            que.push_back(value);

        }
        //  Query the maximum value in the current queue   Directly return to the front of the queue, that is front That's all right. .
        int front() {
    
            return que.front();
        }
    };
public:
    vector<int> maxSlidingWindow(vector<int>& nums, int k) {
    
        MyQueue que;
        vector<int> result;
        for (int i = 0; i < k; i++) {
     //  Before k Put the elements in the queue 
            que.push(nums[i]);
        }
        result.push_back(que.front()); // result  Before recording k The maximum value of the element 
        for (int i = k; i < nums.size(); i++) {
    
            que.pop(nums[i - k]); //  Slide the window to remove the front most element 
            que.push(nums[i]); //  Add the last element before sliding the window 
            result.push_back(que.front()); //  Record the corresponding maximum value 
        }
        return result;
    }
};

347. front K High frequency elements

class Solution {
    
public:
    vector<int> topKFrequent(vector<int>& nums, int k) {
    
        map<int,int> mp;
        vector<int> res;

        for(int num : nums)
        {
    
            mp[num]++;
        }

        vector<pair<int,int>> vmp;

        for(auto it = mp.begin(); it != mp.end(); it++)
        {
    
            vmp.push_back({
    it->first, it->second});
        }

        sort(vmp.begin(), vmp.end(), 
            [](const pair<int, int>& x, const pair<int,int>& y)->int{
    return x.second > y.second;});

        for(int i = 0; i < k; i++)
        {
    
            res.push_back(vmp[i].first);
        }

        return res;
    }
};