APIO 2019 - Bridges

The main idea in this problem is to use square-root decomposition on queries. For convenience, call type 1 queries updates and type 2 queries calculations.

First, split the queries into blocks of about $\sqrt N$ queries. In each block, there are $\mathcal{O}(\sqrt N)$ updates or calculations. For each block:

• Split the bridges into two groups: changed and unchanged.
• If we sort the calculations and unchanged bridges in decreasing order of weight, we can simply use DSU to find which nodes are connected from those bridges alone.

  • These connected nodes are constant for all calculations in the current block
• To handle the updates:

  • Iterate over the queries in the current block (without sorting)
  • If the query is an update, simply update the bridge's weight
  • If the query is a calculation, iterate through each changed bridge and connect the nodes if the weight limit is above the query's weight limit

    • This works because this means the answer for the current query is dependent only on previous updates
    • The key thing here is that we need a way to roll back DSU unions, since the set of "good" bridges may differ from query to query
    • To achieve this, we simply use DSU with path balancing only and keep a stack of previous DSU operations

Implementation

Time Complexity: $\mathcal{O}((Q + M) \log N \sqrt Q )$

However, it is possible to remove the log factor as mentioned in this comment.

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#include <bits/stdc++.h>
#define FOR(i, x, y) for (int i = x; i < y; i++)
typedef long long ll;
using namespace std;

const int B = 1000;

int n, m, q;

stack<int> stck;