How to Compute Kth Largest Element in a Stream?

Design a class to find the kth largest element in a stream. Note that it is the kth largest element in the sorted order, not the kth distinct element.

Your KthLargest class will have a constructor which accepts an integer k and an integer array nums, which contains initial elements from the stream. For each call to the method KthLargest.add, return the element representing the kth largest element in the stream.

Example:
int k = 3;
int[] arr = [4,5,8,2];
KthLargest kthLargest = new KthLargest(3, arr);
kthLargest.add(3); // returns 4
kthLargest.add(5); // returns 5
kthLargest.add(10); // returns 5
kthLargest.add(9); // returns 8
kthLargest.add(4); // returns 8
Note:
You may assume that nums’ length ≥ k-1 and k ≥ 1.

Compute the K-th Largest using Priority Queue

In the constructor, we store the K and construct a priority queue. Then we need to push the elements into the priority queue.

When we add another element to the priority queue, we need to pop extra elements until we have exactly K elements internally in the priority queue.

Adding one element to the priority queue takes O(logN) time complexity.

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class KthLargest {
public:
    KthLargest(int k, vector<int>& nums) {
       this->kth = k;
       for (const auto &n: nums) {
           pq.push(n);
       } 
    }
    
    int add(int val) {
       pq.push(val);
       while (kth < pq.size()) {
           pq.pop();
       }
       return pq.top();
    }
private:
    int kth;
    priority_queue<int, vector<int>, std::greater<int>> pq;
};
 
/**
 * Your KthLargest object will be instantiated and called as such:
 * KthLargest* obj = new KthLargest(k, nums);
 * int param_1 = obj->add(val);
 */

class KthLargest {
public:
    KthLargest(int k, vector<int>& nums) {
       this->kth = k;
       for (const auto &n: nums) {
           pq.push(n);
       } 
    }
    
    int add(int val) {
       pq.push(val);
       while (kth < pq.size()) {
           pq.pop();
       }
       return pq.top();
    }
private:
    int kth;
    priority_queue<int, vector<int>, std::greater<int>> pq;
};

/**
 * Your KthLargest object will be instantiated and called as such:
 * KthLargest* obj = new KthLargest(k, nums);
 * int param_1 = obj->add(val);
 */

–EOF (The Ultimate Computing & Technology Blog) —