Minimum Number of Vertices to Reach All Nodes is a medium problem at leetcode, I hope you have read the problem carefully and have tried solving it. This problem requires you to get the least number of vertices that can make you pass all nodes you have for the graph of your problem. The question is what are these vertices?
You’re given an array of arrays (or a list of lists in a pythonic way) and each subarray contains two numbers entailing the first one to be the node from which you can move and the other one is the node to which you reach. These nodes are called vertices and each path connecting them is called an edge, the whole picture is a graph. In mathematics, graph theory is a study of such graphs and in this problem, we have a specific type of graph called directed graph where edges are connecting vertices asymmetrically meaning all edges are connecting from one vertex to another as you can see from the arrows in the image below:
A directed graph with 10 vertices and 13 edges (source)
How graph works in this problem?
To know how it works, we need to know one thing: in-degree. Let’s see what that means through these examples
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