Queue : April POTD

Day 12/75:

Question: Number of Students Unable to Eat Lunch -(Easy)

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 ith sandwich in the stack (i = 0 is the top of the stack) and students[j] is the preference of the jth student in the initial queue (j = 0 is the front of the queue). Return the number of students that are unable to eat.

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.        

Approach:

1. Create a queue (studentQueue) to represent the line of students.
2. Iterate through the students array and add each student's preference to the queue.
3. Initialize a variable rotation to keep track of how many times we rotate the queue without any student taking a sandwich.
4. Iterate through the queue until it's empty or until we've rotated it without any student taking a sandwich.Check if the top student in the queue prefers the current sandwich.If they do, remove them from the queue and increment the index of the current sandwich (i).Reset the rotation counter.If not, remove the top student from the queue and add them to the end of the queue.Increment the rotation counter.
5. Return the size of the queue, indicating the number of students unable to eat.

CODE:

Time Complexity: O(n^2)

• In the worst case, we iterate through the queue n times, and each time, we may rotate the queue up to n times.
• Therefore, the time complexity is O(n^2).

Space Complexity: O(n)

• We use a queue to store the students' preferences, requiring O(n) space.