# 2255. Count Prefixes of a Given String

Easy
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.

### 解題

func countPrefixes(words []string, s string) int {
m := make(map[string]bool)
ans := 0
for i:=len(s); i>0; i-- {
m[s[:i]] = true
}
for _, word := range words {
if m[word] { ans++ }
}
return ans
}