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LC1894 - Find the Student That Will Replace the Chalk

Posted on In
Leetcode: 116/2658 (51+52+13)

Description

There are n students in a class numbered from 0 to n - 1. The teacher will give each student a problem starting with the student number 0, then the student number 1, and so on until the teacher reaches the student number n - 1. After that, the teacher will restart the process, starting with the student number 0 again.

You are given a 0-indexed integer array chalk and an integer k. There are initially k pieces of chalk. When the student number i is given a problem to solve, they will use chalk[i] pieces of chalk to solve that problem. However, if the current number of chalk pieces is strictly less than chalk[i], then the student number i will be asked to replace the chalk.

Return the index of the student that will replace the chalk.

Example 1:

Input: chalk = [5,1,5], k = 22
Output: 0
Explanation: The students go in turns as follows:

  • Student number 0 uses 5 chalk, so k = 17.
  • Student number 1 uses 1 chalk, so k = 16.
  • Student number 2 uses 5 chalk, so k = 11.
  • Student number 0 uses 5 chalk, so k = 6.
  • Student number 1 uses 1 chalk, so k = 5.
  • Student number 2 uses 5 chalk, so k = 0.
    Student number 0 does not have enough chalk, so they will have to replace it.
    Example 2:

Input: chalk = [3,4,1,2], k = 25
Output: 1
Explanation: The students go in turns as follows:

  • Student number 0 uses 3 chalk so k = 22.
  • Student number 1 uses 4 chalk so k = 18.
  • Student number 2 uses 1 chalk so k = 17.
  • Student number 3 uses 2 chalk so k = 15.
  • Student number 0 uses 3 chalk so k = 12.
  • Student number 1 uses 4 chalk so k = 8.
  • Student number 2 uses 1 chalk so k = 7.
  • Student number 3 uses 2 chalk so k = 5.
  • Student number 0 uses 3 chalk so k = 2.
    Student number 1 does not have enough chalk, so they will have to replace it.

Constraints:

chalk.length == n
1 <= n <= 105
1 <= chalk[i] <= 105
1 <= k <= 109

Solution

  1. Use k to modulo the sum of chalk
  2. Use the modulo result to compare the elements of chalk. If less than zero, the current student needs to replace the chalk
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# O(n) time | O(1) space
class Solution:
    def chalkReplacer(self, chalk: List[int], k: int) -> int:
        summ = sum(chalk)
        k %= summ
        
        for i in range(len(chalk)):
            if k - chalk[i] < 0:
                return i
            else:
                k -= chalk[i]