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;
    }
}

class Solution {
    public List<Integer> addToArrayForm(int[] num, int k) {
        List<Integer> list = new ArrayList<>();
        int qarz=0, temp=0;
        for(int i=num.length-1; i>=0; i--){
            temp=num[i]+k%10+qarz;
            k/=10;
            System.out.println(temp);
            list.add(0, temp%10);
            qarz=temp/10;
        }
        k+=qarz;
        while(k!=0){
            list.add(0, k%10);
            k/=10;
        }
        return list;
    }
}

class Solution {
    public int lengthOfLastWord(String s) {
        int count=0;
        for(int i=s.length()-1; i>=0; i--){
            if(s.charAt(i)==' ' && count!=0) break;
            if(s.charAt(i)!=' ') count++;
        }
        
        return count;
    }
}

class Solution {
    public int[][] kClosest(int[][] points, int k) {
        Arrays.sort(points, (a, b) -> a[0]*a[0]+a[1]*a[1]-b[0]*b[0]-b[1]*b[1]);
        
        return Arrays.copyOfRange(points, 0, k);
    }
}

class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode head = new ListNode(0);
        ListNode temp = head;
        int carry=0;
        while(l1!=null || l2!=null){
            temp.next=new ListNode();
            temp=temp.next;
            temp.val = (l1!=null?l1.val:0) + (l2!=null?l2.val:0)+carry;
            carry=temp.val/10;
            temp.val %= 10;
            if(l1!=null) l1=l1.next;
            if(l2!=null) l2=l2.next;
        }
        if(carry>0)
            temp.next = new ListNode(carry);
        return head.next;
    }
}

select id, 'Root' as type from tree where p_id is null
union
select id, 'Inner' as type from tree
where id in (select distinct p_id from tree where p_id is not null) and p_id is not null
union
select id, 'Leaf' as type from tree
where id not in(select distinct p_id from tree where p_id is not null) and p_id is not null

select distinct author_id as id from views where author_id=viewer_id order by author_id

select activity_date as day, count(distinct user_id) as active_users from activity
where datediff('2019-07-27', activity_date)<30
group by activity_date

select user_id, count(distinct follower_id) as followers_count
from followers group by user_id;

select actor_id, director_id 
from (select actor_id, director_id, count(*) as total from actordirector group by actor_id, director_id) as a where total>=3