LeetCode 2255. Count Prefixes of a Given String Solution in Java, C++, Python & More | Explanation + Code

Description

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.

Solutions

Solution 1: Traversal Counting

We directly traverse the array words, and for each string w, we check if s starts with w as a prefix. If it does, we increment the answer by one.

After the traversal, we return the answer.

The time complexity is O(m × n), where m and n are the lengths of the array words and the string s, respectively. The space complexity is O(1).

PythonJavaC++GoTypeScriptRustC#

class Solution: def countPrefixes(self, words: List[str], s: str) -> int: return sum(s.startswith(w) for w in words)(code-box)

class Solution { public int countPrefixes(String[] words, String s) { int ans = 0; for (String w : words) { if (s.startsWith(w)) { ++ans; } } return ans; } }(code-box)

class Solution { public: int countPrefixes(vector<string>& words, string s) { int ans = 0; for (auto& w : words) { ans += s.starts_with(w); } return ans; } };(code-box)

func countPrefixes(words []string, s string) (ans int) { for _, w := range words { if strings.HasPrefix(s, w) { ans++ } } return }(code-box)

function countPrefixes(words: string[], s: string): number { return words.filter(w => s.startsWith(w)).length; }(code-box)

impl Solution { pub fn count_prefixes(words: Vec<String>, s: String) -> i32 { words.iter().filter(|w| s.starts_with(w.as_str())).count() as i32 } }(code-box)

public class Solution { public int CountPrefixes(string[] words, string s) { return words.Count(w => s.StartsWith(w)); } }(code-box)