LESSON 238 OF 671 · SETS

Every Letter in Java: Check a Sentence With a HashSet

You get a string sentence made of lowercase letters and spaces.

Return true if every letter from a to z appears in it at least once. Otherwise, return false.

Spaces are not letters, so they don't count. A letter may appear many times.

Example 1:

Input: sentence = "my jumpy dog fixed a big quiz with seven lucky parrots"
Output: true
Explanation: All 26 letters appear at least once.

Example 2:

Input: sentence = "six jumpy kittens quiz the brave fox and a wild hog"
Output: false
Explanation: The letter c is missing.
The other 25 letters all appear.

Example 3:

Input: sentence = "abc abc abc"
Output: false
Explanation: Only 3 different letters appear: a, b and c.

Constraints:

sentence has 1 to 1,000 characters.
It has only lowercase English letters and spaces.
It has at least one letter and doesn't start or end with a space.

STEP 1 OF 5

Guided

Ctrl + Enter to check

Every Letter in Java, explained

Problem overview

The method usesEveryLetter receives a string sentence made of lowercase letters and spaces. It returns true if every letter from a to z appears in it at least once, and false otherwise. Spaces are not letters, and a letter may appear many times.

The sentence "my jumpy dog fixed a big quiz with seven lucky parrots" has all 26 letters, so the answer is true. The sentence "six jumpy kittens quiz the brave fox and a wild hog" has 25 of them and no c, so the answer is false. The sentence "abc abc abc" has only three different letters, so the answer is false.

Approach

Collect the letters of the sentence in a HashSet. A set keeps each value once, however many times it is added, so after the loop the set holds exactly the different letters that appeared. Since the sentence has only lowercase English letters, there are 26 possible ones. If the set holds 26 letters, none is missing.

The one thing to watch is the space. It is a character too, so it would go in the set like any other. The code must keep spaces out.

Step by step

Create an empty HashSet<Character> named letters.
Loop over the characters of the sentence with a char named c, using sentence.toCharArray().
Inside the loop, check that c is not a space.
Inside the if, add c to letters.
After the loop, return whether letters.size() is 26.

The solution in Java

class Solution {
    public boolean usesEveryLetter(String sentence) {
        HashSet<Character> letters = new HashSet<>();
        for (char c : sentence.toCharArray()) {
            if (c != ' ') {
                letters.add(c);
            }
        }
        return letters.size() == 26;
    }
}

Some Java details:

A set holds objects, so its type is HashSet<Character>, not HashSet<char>. Java wraps each char into a Character when you call add.
A single character is written in single quotes, ' '. Double quotes would make a string, and c != " " would not compile.
letters.size() == 26 is already true or false, so the method returns it directly.

Following the second example: the loop adds the letters it meets, skipping the spaces, and ends with 25 different letters in the set. Only c is missing, so letters.size() == 26 is false. In the first example the set ends with all 26 letters, and the comparison is true.

Another way to think about it is that the set shrinks the sentence to its alphabet. Once you have the alphabet, the question becomes a simple count, and you don't need to check the letters one at a time against a to z.

Edge cases and common mistakes

Letting the space in. Without the if, the first example's set would hold 27 characters: 26 letters and the space. A full sentence would then fail the == 26 test. The same bug would make a sentence with 25 letters and a space report 26 and wrongly return true.
Counting instead of collecting. The length of the sentence says nothing about the number of different letters.
Repeated letters. "abc abc abc" adds nine letters, but the set ends with three.

Testing

Running the solution on the three examples gives:

true
false
false

The tests print booleans in lowercase.

Keep going with Java500+ step-by-step lessons, from your first line of code to coding interviews. Free to start.

Start learning free