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###### tags: `Leetcode`
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# Leetcode 1700. Number of Students Unable to Eat Lunch

[link](https://leetcode.com/problems/number-of-students-unable-to-eat-lunch/)

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The school cafeteria offers circular and square sandwiches at lunch break, referred to by numbers 0 and 1 respectively. All students stand in a queue. Each student either prefers square or circular sandwiches.

The number of sandwiches in the cafeteria is equal to the number of students. The sandwiches are placed in a stack. At each step:

- If the student at the front of the queue prefers the sandwich on the top of the stack, they will take it and leave the queue.
- Otherwise, they will leave it and go to the queue's end.
This continues until none of the queue students want to take the top sandwich and are thus unable to eat.

You are given two integer arrays students and sandwiches where sandwiches[i] is the type of the i​​​​​​th sandwich in the stack (i = 0 is the top of the stack) and students[j] is the preference of the j​​​​​​th student in the initial queue (j = 0 is the front of the queue). Return the number of students that are unable to eat.
#### Example 1:

Input: students = [1,1,0,0], sandwiches = [0,1,0,1]
Output: 0 
Explanation:
- Front student leaves the top sandwich and returns to the end of the line making students = [1,0,0,1].
- Front student leaves the top sandwich and returns to the end of the line making students = [0,0,1,1].
- Front student takes the top sandwich and leaves the line making students = [0,1,1] and sandwiches = [1,0,1].
- Front student leaves the top sandwich and returns to the end of the line making students = [1,1,0].
- Front student takes the top sandwich and leaves the line making students = [1,0] and sandwiches = [0,1].
- Front student leaves the top sandwich and returns to the end of the line making students = [0,1].
- Front student takes the top sandwich and leaves the line making students = [1] and sandwiches = [1].
- Front student takes the top sandwich and leaves the line making students = [] and sandwiches = [].
Hence all students are able to eat.

#### Example 2:

Input: students = [1,1,1,0,0,1], sandwiches = [1,0,0,0,1,1]
Output: 3

#### Constraints:

- 1 <= students.length, sandwiches.length <= 100
- students.length == sandwiches.length
- sandwiches[i] is 0 or 1.
- students[i] is 0 or 1.

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The code creates two deques, q and s, initialized with the students and sandwiches lists, respectively.
It initializes a count variable to 0 to keep track of the number of iterations.

It enters a while loop that continues until the number of iterations (count) is equal to the length of the students list.

Inside the loop, it checks if the deques q and s are equal. If they are, it means that the student at the front of the line (represented by q.popleft()) prefers the sandwich at the front of the queue (represented by s.popleft()), and they can be served. In this case, both the student and the sandwich are removed from the respective deques, and the count is reset to 0.

If the deques q and s are not equal, it means the student at the front of the line does not prefer the sandwich at the front of the queue. In this case, the student is moved to the end of the line (q.append(q.popleft())), and the count is incremented by 1.

After the loop ends, the function returns the length of the students list, which represents the number of students who did not get their preferred sandwiches.

#### Solution 1

class Solution:
    def countStudents(self, students: List[int], sandwiches: List[int]) -> int:
        q = deque(students)
        s = deque(sandwiches)
        count = 0
        while len(q) > count:
            if q[0] == s[0]:
                q.popleft()
                s.popleft()
                count = 0

            else:
                q.append(q.popleft())
                count += 1
        
        return len(q)
        

O(T): O(N)
O(S): O(N)