O(N 1.5 ) divide-and-conquer technique for Minimum Spanning Tree problem Step 1: Divide the graph into  N sub-graph by clustering. Step 2: Solve each sub-problem separately using Prim's algorithm (quadratic complexity) Step 3: Merge the sub-solutions: Construct a meta graph where each node corresponds to one cluster Solve MST for the meta graph Add links from the meta graph to the original graph to complete the solution

Example 1: Algorithm finds sub-optimal solution

Step 1: Divide the graph into  N sub-graph by clustering

Step 2: Solve each sub-problem by Prim’s algorithm

Step 3.1 (a): Select center point for each cluster

Step 3.1 (b): Connect the nodes of this meta graph

Step 3.1 (c): Set the weights based on shortest distances 1 3 4

Step 3.2: Solve MST for the meta graph 1 3 4

Step 3.3: Select the corresponding links

Step 3.3: Add links from the MST of the meta graph Total weight = 21

Better solution: Total weight = 20 Optimality of the solution? Add Remove

Total weight = 20 Example 2: Algorithm finds optimal solution