Selection algorithms on sorted matrix

Question

this is a google interview question :

Given a N*N Matrix. All rows are sorted, and all columns are sorted. Find the Kth Largest element of the matrix.

doing

Answer 1

The following is my C++ solution, which is based on a min heap. When a cell in the matrix is on the top of the min heap, the number at the right and/or the downside will be inserted into the heap.

#include 
#include 
#include 

using namespace std;

struct Entry {
    int value;
    int x;
    int y;

    bool operator < (const Entry& other) {
        return this->value > other.value;
    }
};

bool getKthNumber(int* matrix, int row, int col, int k, int* result){
    if(matrix == NULL || row <= 0 || col <= 0 || result == NULL)
        return false;
    if(k <= 0 || k > row * col)
        return false;

    vector minHeap;
    Entry first = {matrix[0], 0, 0};
    minHeap.push_back(first);
    make_heap(minHeap.begin(), minHeap.end());

    for(int i = 0; i < k; ++i){
        first = minHeap[0];
        int x = first.x;
        int y = first.y;
        if(first.y == 0 && first.x < row - 1){
            Entry next = {matrix[(x + 1) * col], x + 1, y};
            minHeap.push_back(next);
            push_heap(minHeap.begin(), minHeap.end());
        }
        if(first.y < col - 1){
            Entry next = {matrix[x * col + y + 1], x, y + 1};
            minHeap.push_back(next);
            push_heap(minHeap.begin(), minHeap.end());
        }

        pop_heap(minHeap.begin(), minHeap.end());
        minHeap.pop_back();
    }

    *result = first.value;
    return true;
}