Leetcode-Easy 806. Number of Lines To Write String

Example :
Input: 
widths = [10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10]
S = "abcdefghijklmnopqrstuvwxyz"
Output: [3, 60]
Explanation: 
All letters have the same length of 10. To write all 26 letters,
we need two full lines and one line with 60 units.

class Solution:
    def numberOfLines(self, widths, S):
        """
        :type widths: List[int]
        :type S: str
        :rtype: List[int]
        """
        letter_list=list("abcdefghijklmnopqrstuvwxyz")
        length=0
        line=1
        for s in S:
            length+=widths[letter_list.index(s)]
            if length>100:
                line+=1
                length=widths[letter_list.index(s)]
        return [line,length]