Description

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.

Code

Time Complexity: , Space Complexity:

KEY:只要當前的 sandwiches[i] 對應到的數字在 students 中還沒有被對應到,就可以透過旋轉 students 使之對應到,反之,若 students 中已經都對應完了,無論怎麼旋轉,都無法完成配對。

students = [1,1,1,0,0,1], sandwiches = [1,0,0,0,1,1] 為例,sandwiches 的1,0,0 都可以被對應到,students 的變化如下:[1,1,1,0,0,1] -> [1,1,0,0,1] -> [0,0,1,1,1] -> [1,1,1],至此,要對應的是 sandwiches 的 0(index 3),但是 students 中已經沒有任何的 0,因此無法繼續對應。

class Solution {
public:
    int countStudents(vector<int>& A, vector<int>& B) {
        int count[] = {0, 0}, n = A.size(), k;
        for (int a : A)
            count[a]++;
        for (k = 0; k < n && count[B[k]] > 0; ++k)
            count[B[k]]--;
        return n - k;
   }
};

Source