#3613

Medium

auto_awesomeBinary search over the valid answer space

LeetCode Problem Workspace

Minimize Maximum Component Cost

Minimize Maximum Component Cost leverages binary search over edge weights to optimize the cost of graph components after edge removal.

binary search sort union find graph

Problem Statement

You are given an undirected connected graph with n nodes labeled from 0 to n - 1 and a 2D integer array edges, where edges[i] = [ui, vi, wi] denotes an undirected edge between node ui and node vi with weight wi, and an integer k. Your goal is to minimize the maximum edge weight in the components of the graph after removing some edges.

You are allowed to remove any number of edges from the graph such that the resulting graph has at most k connected components. The cost of each component is defined as the maximum edge weight within that component. If a component has no edges, its cost is 0.

Examples

Example 1

Input: n = 5, edges = [[0,1,4],[1,2,3],[1,3,2],[3,4,6]], k = 2

Output: 4

Example 2

Input: n = 4, edges = [[0,1,5],[1,2,5],[2,3,5]], k = 1

Output: 5

Constraints

1 <= n <= 5 * 104
0 <= edges.length <= 105
edges[i].length == 3
0 <= ui, vi < n
1 <= wi <= 106
1 <= k <= n
The input graph is connected.

Solution Approach

Binary Search on Maximum Edge Weight

The solution begins with sorting the edges by their weights. We then perform a binary search on the possible values for the maximum edge weight in any component. For each candidate weight, we check if it is possible to divide the graph into at most k components with edges having weights less than or equal to this candidate. This involves using a Union-Find data structure to efficiently track connected components.

Union-Find for Component Formation

Union-Find is used to manage the connectivity between nodes as edges are removed based on the current candidate weight. Each time an edge with weight less than or equal to the candidate is considered, we unite the two nodes. If the graph is split into more than k components, the candidate weight is adjusted. This helps find the minimum maximum weight that satisfies the k-component condition.

Edge Sorting and Complexity

Sorting the edges allows the binary search to work efficiently, as we only need to examine edge weights in increasing order. This makes the approach both time-efficient and suitable for large inputs. The binary search is combined with the Union-Find operations to determine the optimal solution within the given constraints.

Complexity Analysis

Metric	Value
Time	Depends on the final approach
Space	Depends on the final approach

The time complexity is dominated by the binary search and the sorting of edges. Sorting the edges takes O(E log E), where E is the number of edges. For each binary search iteration, we perform a union-find operation, which takes nearly O(α(N)) time where α is the inverse Ackermann function. Thus, the overall time complexity is O(E log E + E α(N)). The space complexity is O(N + E) for the Union-Find data structure and the edge list.

What Interviewers Usually Probe

The candidate efficiently handles the graph's connectivity with Union-Find.
The candidate demonstrates understanding of binary search applied to optimization problems.
The candidate uses edge sorting as a natural step to streamline the solution process.

Common Pitfalls or Variants

Common pitfalls

Overlooking the need to sort the edges before applying binary search.
Misunderstanding the edge weights when performing Union-Find operations.
Failing to handle the k-component constraint correctly, leading to an incorrect solution.

Follow-up variants

Increase the number of components allowed (k) and observe how it affects the optimization process.
Change the graph from being connected to having multiple disjoint subgraphs and analyze the approach.
Allow negative edge weights and evaluate how the algorithm adapts to this new scenario.

FAQ

What is the main pattern used to solve the Minimize Maximum Component Cost problem?

The main pattern used is binary search over the possible maximum edge weights, combined with Union-Find to manage connected components in the graph.

How does sorting the edges help in this problem?

Sorting the edges allows binary search to efficiently explore the valid range of edge weights, ensuring that the solution is both correct and optimal.

Can this approach be applied to graphs with negative edge weights?

No, this approach assumes positive edge weights. Modifications would be required to handle negative weights effectively.

Why is Union-Find important in this problem?

Union-Find is crucial for efficiently tracking connected components and ensuring that the graph can be split into at most k components while minimizing the maximum edge weight.

How does GhostInterview assist in solving graph problems like Minimize Maximum Component Cost?

GhostInterview helps by providing structured guidance on applying binary search, optimizing graph connectivity, and using Union-Find to solve such problems efficiently.

terminal

Solution

Solution 1: Sorting + Union-Find

If $k = n$, it means all edges can be removed. In this case, all connected components are isolated nodes, and the maximum cost is 0.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

class Solution:
    def minCost(self, n: int, edges: List[List[int]], k: int) -> int:
        def find(x: int) -> int:
            if p[x] != x:
                p[x] = find(p[x])
            return p[x]

        if k == n:
            return 0
        edges.sort(key=lambda x: x[2])
        cnt = n
        p = list(range(n))
        for u, v, w in edges:
            pu, pv = find(u), find(v)
            if pu != pv:
                p[pu] = pv
                cnt -= 1
                if cnt <= k:
                    return w
        return 0

Continue Practicing

#3608 minimum time for k connected components #3600 maximize spanning tree stability with upgrades #3532 path existence queries in a graph i