884. Uncommon Words from Two Sentences

Difficulty: Easy

Topics: Hash Table, String, Counting

A sentence is a string of single-space separated words where each word consists only of lowercase letters.

A word is uncommon if it appears exactly once in one of the sentences, and does not appear in the other sentence.

Given two sentences s1 and s2, return a list of all the uncommon words. You may return the answer in any order.

Example 1:

Input: s1 = "this apple is sweet", s2 = "this apple is sour"
Output: ["sweet","sour"]
Explanation: The word "sweet" appears only in s1, while the word "sour" appears only in s2.

Example 2:

Input: s1 = "apple apple", s2 = "banana"
Output: ["banana"]

Constraints:

1 <= s1.length, s2.length <= 200
s1 and s2 consist of lowercase English letters and spaces.
s1 and s2 do not have leading or trailing spaces.
All the words in s1 and s2 are separated by a single space.

Solution:

We can follow these steps:

Break the sentences into words: Split both s1 and s2 into individual words.
Count the occurrences of each word: Use an associative array (hash table) to count how many times each word appears across both sentences.
Filter uncommon words: Find words that appear exactly once in the combined set of words from both sentences and do not appear in both sentences.

Approach:

Use the explode function to split the sentences into arrays of words.
Use an associative array to count the frequency of each word.
Return the words that have a count of 1.

Let's implement this solution in PHP: 884. Uncommon Words from Two Sentences

<?php
/**
 * @param String $s1
 * @param String $s2
 * @return String[]
 */
function uncommonFromSentences($s1, $s2) {
    ...
    ...
    ...
    /**
     * go to ./solution.php
     */
}

// Test cases
$s1 = "this apple is sweet";
$s2 = "this apple is sour";
print_r(uncommonFromSentences($s1, $s2)); // Output: ["sweet", "sour"]

$s1 = "apple apple";
$s2 = "banana";
print_r(uncommonFromSentences($s1, $s2)); // Output: ["banana"]
?>

Explanation:

Splitting the sentences: We use explode(' ', $s1) and explode(' ', $s2) to split the sentences into arrays of words.
Counting occurrences: We loop through both arrays of words, incrementing the count of each word in the associative array $wordCount.
Filtering uncommon words: We then check for words that have a count of exactly 1 in the $wordCount array and add them to the result array.

Example Walkthrough:

For s1 = "this apple is sweet" and s2 = "this apple is sour", after counting, the associative array $wordCount will look like:

  Array (
    [this] => 2
    [apple] => 2
    [is] => 2
    [sweet] => 1
    [sour] => 1
  )

The words with a count of 1 are sweet and sour, so the result is ["sweet", "sour"].

Time Complexity:

O(n + m), where n is the number of words in s1 and m is the number of words in s2. This is because we iterate through all the words in both sentences once to count the occurrences.

Space Complexity:

O(n + m), where n and m are the number of words in s1 and s2. We store all the words in an associative array to track their counts.

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