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2085. Count Common Words With One Occurrence
Description
Given two string arrays words1
and words2
, return the number of strings that appear exactly once in each of the two arrays.
Example 1:
Input: words1 = ["leetcode","is","amazing","as","is"], words2 = ["amazing","leetcode","is"] Output: 2 Explanation:  "leetcode" appears exactly once in each of the two arrays. We count this string.  "amazing" appears exactly once in each of the two arrays. We count this string.  "is" appears in each of the two arrays, but there are 2 occurrences of it in words1. We do not count this string.  "as" appears once in words1, but does not appear in words2. We do not count this string. Thus, there are 2 strings that appear exactly once in each of the two arrays.
Example 2:
Input: words1 = ["b","bb","bbb"], words2 = ["a","aa","aaa"] Output: 0 Explanation: There are no strings that appear in each of the two arrays.
Example 3:
Input: words1 = ["a","ab"], words2 = ["a","a","a","ab"] Output: 1 Explanation: The only string that appears exactly once in each of the two arrays is "ab".
Constraints:
1 <= words1.length, words2.length <= 1000
1 <= words1[i].length, words2[j].length <= 30
words1[i]
andwords2[j]
consists only of lowercase English letters.
Solutions
Solution 1: Hash Table + Counting
We can use two hash tables, $cnt1$ and $cnt2$, to count the occurrences of each string in the two string arrays respectively. Then, we traverse one of the hash tables. If a string appears once in the other hash table and also appears once in the current hash table, we increment the answer by one.
The time complexity is $O(n + m)$, and the space complexity is $O(n + m)$. Where $n$ and $m$ are the lengths of the two string arrays respectively.

class Solution { public int countWords(String[] words1, String[] words2) { Map<String, Integer> cnt1 = new HashMap<>(); Map<String, Integer> cnt2 = new HashMap<>(); for (var w : words1) { cnt1.merge(w, 1, Integer::sum); } for (var w : words2) { cnt2.merge(w, 1, Integer::sum); } int ans = 0; for (var e : cnt1.entrySet()) { if (e.getValue() == 1 && cnt2.getOrDefault(e.getKey(), 0) == 1) { ++ans; } } return ans; } }

class Solution { public: int countWords(vector<string>& words1, vector<string>& words2) { unordered_map<string, int> cnt1; unordered_map<string, int> cnt2; for (auto& w : words1) { ++cnt1[w]; } for (auto& w : words2) { ++cnt2[w]; } int ans = 0; for (auto& [w, v] : cnt1) { ans += v == 1 && cnt2[w] == 1; } return ans; } };

class Solution: def countWords(self, words1: List[str], words2: List[str]) > int: cnt1 = Counter(words1) cnt2 = Counter(words2) return sum(v == 1 and cnt2[w] == 1 for w, v in cnt1.items())

func countWords(words1 []string, words2 []string) (ans int) { cnt1 := map[string]int{} cnt2 := map[string]int{} for _, w := range words1 { cnt1[w]++ } for _, w := range words2 { cnt2[w]++ } for w, v := range cnt1 { if v == 1 && cnt2[w] == 1 { ans++ } } return }

function countWords(words1: string[], words2: string[]): number { const cnt1 = new Map<string, number>(); const cnt2 = new Map<string, number>(); for (const w of words1) { cnt1.set(w, (cnt1.get(w) ?? 0) + 1); } for (const w of words2) { cnt2.set(w, (cnt2.get(w) ?? 0) + 1); } let ans = 0; for (const [w, v] of cnt1) { if (v === 1 && cnt2.get(w) === 1) { ++ans; } } return ans; }