## DEV Community

Abhishek Chaudhary

Posted on

# Count Prefixes of a Given String

You are given a string array `words` and a string `s`, where `words[i]` and `s` comprise only of lowercase English letters.

Return the number of strings in `words` that are a prefix of `s`.

A prefix of a string is a substring that occurs at the beginning of the string. A substring is a contiguous sequence of characters within a string.

Example 1:

Input: words = ["a","b","c","ab","bc","abc"], s = "abc"
Output: 3
Explanation:
The strings in words which are a prefix of s = "abc" are:
"a", "ab", and "abc".
Thus the number of strings in words which are a prefix of s is 3.

Example 2:

Input: words = ["a","a"], s = "aa"
Output: 2
Explanation:
Both of the strings are a prefix of s.
Note that the same string can occur multiple times in words, and it should be counted each time.

Constraints:

• `1 <= words.length <= 1000`
• `1 <= words[i].length, s.length <= 10`
• `words[i]` and `s` consist of lowercase English letters only.

SOLUTION:

``````class Solution:
def countPrefixes(self, words: List[str], s: str) -> int:
n = len(s)
prefixes = set()
for i in range(1, n + 1):