class Solution {
    public ListNode mergeNodes(ListNode head) {
        ListNode slow = head.next;
        ListNode fast = head.next;
        int sum=0;
        while(fast!=null){
            sum+=fast.val;
            if(fast.val==0){
                slow.val=sum;
                slow.next=fast.next;
                slow=slow.next;
                sum=0;
            }
            fast=fast.next;
        }
        return head.next;
    }
}

class Solution {
    int sum=0;
    public int sumEvenGrandparent(TreeNode root) {
        if(root==null) return 0;
        helper(root, null, null);
        return sum;
    }
    
    public void helper(TreeNode node, TreeNode p, TreeNode gp){
        if(node == null) return;
        
        if(gp!=null && gp.val%2==0){
            sum+=node.val;
        }
        
        helper(node.left, node, p);
        helper(node.right, node, p);
    }
}

select sell_date,
count(distinct product) as num_sold,
group_concat(distinct product order by product) as products
from activities
group by sell_date

class Solution {
    int sum=0;
    public TreeNode bstToGst(TreeNode root) {
        if(root==null) return null;
        bstToGst(root.right);
        sum+=root.val;
        root.val=sum;
        bstToGst(root.left);
        
        return root;
    }
}

class Solution {
    int sum=0;
    public TreeNode convertBST(TreeNode root) {
        if(root==null) return null;
        convertBST(root.right);
        sum+=root.val;
        root.val=sum;
        convertBST(root.left);
        
        return root;
    }
}

select user_id,
max(time_stamp) as last_stamp
from logins
where year(time_stamp)=2020
group by user_id

select u.name, ifnull(sum(r.distance), 0) as travelled_distance
from users u
left join rides r on u.id=r.user_id
group by u.name
order by travelled_distance desc, u.name asc;

class Solution {
    public int[] processQueries(int[] queries, int m) {
        List<Integer> list = new ArrayList<>();
        for(int i=1; i<=m; i++)
            list.add(i);
        
        int[] ans = new int[queries.length];
        for(int i=0; i<ans.length; i++){
            ans[i]=list.indexOf(queries[i]);
            list.remove(ans[i]);
            list.add(0, queries[i]);
        }
        
        return ans;
    }
}

class Solution {
    public int strStr(String haystack, String needle) {
        if(haystack.length()<needle.length()) return -1;
        for(int i=0; i<haystack.length(); i++){
            for(int j=0; ; j++){
                if(i+j==haystack.length()) return -1;
                if(haystack.charAt(i+j) != needle.charAt(j)) break;
                if(j==needle.length()-1) return i;
            }
        }
        return -1;
    }
}

select name from customer
where referee_id <> 2 or referee_id is null

class Solution {
    public int minBitFlips(int start, int goal) {
        int ans, count=0;
        ans=start^goal;
        while(ans>0){
            if((ans&1)==1) count++;
            ans>>=1;
        }
        return count;
    }
}

select employee_id from employees
where employee_id not in (select employee_id from salaries)
union
select employee_id from salaries
where employee_id not in (select employee_id from employees)
order by employee_id;

class Solution {
    public int pairSum(ListNode head) {
        int l=length(head), max=0, index;
        int[] arr = new int[l/2];
        for(int i=0; i<l; i++){
            index=i<l/2 ? i : l-i-1;
            arr[index] += head.val;
            head=head.next;
            max=Math.max(max, arr[index]);
        }
        return max;
    }
    public int length(ListNode node){
        int count=0;
        while(node!=null){
            count++;
            node=node.next;
        }
        return count;
    }
}

class Solution {
    public int[] findingUsersActiveMinutes(int[][] logs, int k) {
        int[] ans = new int[k];
        Map<Integer, Set<Integer>> map = new HashMap<>();
        for(int[] log: logs){
            map.putIfAbsent(log[0], new HashSet<>());
            map.get(log[0]).add(log[1]);
        }
        for(Map.Entry<Integer, Set<Integer>> entry : map.entrySet()){
            ans[entry.getValue().size()-1]++;
        }
        return ans;
    }
}

select player_id, min(event_date) as first_login from activity group by player_id;

class Solution {
    public int triangularSum(int[] nums) {
        for(int i=nums.length-1; i>0; i--){
            for(int j=0; j<i; j++){
                nums[j] = (nums[j] + nums[j+1])%10;
            }
        }
        return nums[0];
    }
}

select user_id, concat(upper(substr(name, 1, 1)), lower(substr(name, 2)))
as name from users
order by user_id;

select * from patients
where conditions like '% diab1%' or conditions like 'diab1%';

class Solution {
    public boolean repeatedSubstringPattern(String s) {
        for(int i=0; i<s.length()/2; i++){
            if(s.length()%(i+1)==0 && check(i, s))
                return true;
        }
        return false;
    }
    public boolean check(int i, String s){
        StringBuilder temp = new StringBuilder(s.substring(0, i+1));
        int count=s.length()/(i+1);
        while(--count >0){
            temp.append(s.substring(0, i+1));
        }
        if(temp.toString().equals(s)) return true;
        return false;
    }
}

class Solution {
    public int evalRPN(String[] tokens) {
        Stack<String> stack = new Stack<>();
        for(String t: tokens){
            if(!stack.isEmpty() && t.charAt(0)<48 && t.length()==1){
                int num2=Integer.parseInt(stack.pop());
                int num1=Integer.parseInt(stack.pop());
                stack.push(String.valueOf(calculate(num1, num2, t.charAt(0))));
            }else{
                stack.push(t);
            }
        }
        return Integer.parseInt(stack.peek());
    }
    public int calculate(int num1, int num2, char ch){
        switch(ch){
            case '+': return num1+num2;
            case '-': return num1-num2;
            case '*': return num1*num2;
            case '/': return num1/num2;
        }
        return 0;
    }
}