Hey, so this is very similar code I wrote with extra comments to explain what's going on, but for some reason, this one has about 3 WAs, even when there's very little difference between your and my solution. Still, I am posting this so hopefully someone can benefit from the extra explanation of the code comments.

Code:

#include <bits/stdc++.h>
 
using namespace std;
 
size_t N, K;
 
size_t solve (vector<uint>& a, vector<uint>& b, size_t k) {
    priority_queue<uint> A, B;
    for (auto ele: a) A.push (ele);
    for (auto ele: b) B.push (-ele); // serves as min-heap
 
    for (; k != 0; k--) {
        // here b contains the biggest book, so we swap
        // smaller books from that shelf