Today I have done leetcode's October LeetCoding Challenge with cpp.

October LeetCoding Challenge 19

Description

Top K Frequent Words

Given an array of strings words and an integer k, return the k most frequent strings.

Return the answer sorted by the frequency from highest to lowest. Sort the words with the same frequency by their lexicographical order.

Example 1:

Input: words = ["i","love","leetcode","i","love","coding"], k = 2
Output: ["i","love"]
Explanation: "i" and "love" are the two most frequent words.
Note that "i" comes before "love" due to a lower alphabetical order.

Example 2:

Input: words = ["the","day","is","sunny","the","the","the","sunny","is","is"], k = 4
Output: ["the","is","sunny","day"]
Explanation: "the", "is", "sunny" and "day" are the four most frequent words, with the number of occurrence being 4, 3, 2 and 1 respectively.

Constraints:

• 1 <= words.length <= 500
• 1 <= words[i].length <= 10
• words[i] consists of lowercase English letters.
• k is in the range [1, The number of unique words[i]]

Follow-up: Could you solve it in O(n log(k)) time and O(n) extra space?

Solution

class Solution {
  using psi = pair<string, int>;
public:
  vector<string> topKFrequent(vector<string>& words, int k) {
    map<string, int> count;
    for(const auto &word : words) {
      count[word] += 1;
    }
    vector<psi> container(count.begin(), count.end());
    sort(container.begin(), container.end(), [](const psi &a, const psi &b) {
      return a.second > b.second || (a.second == b.second && a.first < b.first);
    });
    vector<string> answer;
    transform(container.begin(), container.begin() + k, back_inserter(answer), [](const psi &a) {
      return a.first;
    });
    return move(answer);
  }
};

// Accepted
// 110/110 cases passed (15 ms)
// Your runtime beats 85.73 % of cpp submissions
// Your memory usage beats 88.37 % of cpp submissions (12.5 MB)