1481.Least Number of Unique Integers after K Removals

Tags: Medium Array Sort

Links: https://leetcode.com/problems/least-number-of-unique-integers-after-k-removals/

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Given an array of integers arr and an integer k. Find the least number of unique integers after removing exactly k elements**.**

Example 1:

Input: arr = [5,5,4], k = 1
Output: 1
Explanation: Remove the single 4, only 5 is left.

Example 2:

Input: arr = [4,3,1,1,3,3,2], k = 3
Output: 2
Explanation: Remove 4, 2 and either one of the two 1s or three 3s. 1 and 3 will be left.

Constraints:

• 1 <= arr.length <= 10^5
• 1 <= arr[i] <= 10^9
• 0 <= k <= arr.length

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贪心+哈希表+排序。

用哈希表统计不同数字的个数，用数组seq存储每个数字对应的个数，然后对seq进行排序，为了让最后不同的数字种类最少，肯定是从数量少的开始删除才会得到最优解。

最坏的情况是数周内的数字都不相同，时间复杂度$O(n \log n)$。

class Solution {
    vector<int> seq;
    unordered_map<int, int> um;
public:
    int findLeastNumOfUniqueInts(vector<int>& arr, int k) {
        std::ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        cout.tie(NULL);

        for (const auto & e : arr) ++um[e];
        for (const auto & e : um) seq.push_back(e.second);

        sort(seq.begin(), seq.end());
        int n = seq.size();
        int cnt = 0;
        for (int i = 0; i < n; ++i) {
            if (seq[i] <= k) {
                ++cnt;
                k -= seq[i];
            }
            else {
                k = 0;
            }

            if (k == 0) break;
        }

        return n - cnt;
    }
};