Algorithm of the Week: Graph Breadth First Search
Join the DZone community and get the full member experience.
Join For FreeSince we already know how to represent graphs, we can go further for some very simple approaches of walking through them. Passing by all the vertices of a graph is a fundamental technique for most of the graph algorithms, such as finding shortest/longest paths, etc.
First thing to note is that graphs are not trees, in most of the cases, so walking through them can’t start from a root, as we do with trees. What we must do first is to decide from where to start – in other words – choosing a starting vertex.
After that we need to know how to proceed. There are two approaches
mostly known as “breadth first” and “depth first” search. While depth
first search start from a vertex and goes as far as possible, then walks
back and passes through vertices that haven’t been visited yet, breath
first search is an approach of passing through all the neighbors of the
node first, and then go to the next level.
Overview
We can thing of breadth first search as a “wave” walk through the graph. In other words we go level by level, as shown on the picture below.
Initially we mark all vertices as unvisited. A common approach is to create an empty queue where we put the vertices level by level, starting with the initial vertex.
Code
This simple approach is fairly easy to implement. Here’s the PHP implementation in few lines of code.
<?php $g = array( 0 => array(0, 1, 1, 0, 0, 0), 1 => array(1, 0, 0, 1, 0, 0), 2 => array(1, 0, 0, 1, 0, 0), 3 => array(0, 1, 1, 0, 1, 0), 4 => array(0, 0, 0, 1, 0, 1), 5 => array(0, 0, 0, 0, 1, 0), ); function init(&$visited, &$graph) { foreach ($graph as $key => $vertex) { $visited[$key] = 0; } } function breadth_first(&$graph, $start, $visited) { // create an empty queue $q = array(); // initially enqueue only the starting vertex array_push($q, $start); $visited[$start] = 1; echo $start . "\n"; while (count($q)) { $t = array_shift($q); foreach ($graph[$t] as $key => $vertex) { if (!$visited[$key] && $vertex == 1) { $visited[$key] = 1; array_push($q, $key); echo $key . "\t"; } } echo "\n"; } } $visited = array(); init($visited, $g); breadth_first($g, 2, $visited);
Complexity
The complexity of this algorithm clearly is O(n^{2}).
Application
As I said breadth first and depth first searches are used in many practical cases, as finding shortest/minimal paths etc. That is why understanding these basic principles of walking through a graph is crucial for other, more complex, graph algorithms.
Opinions expressed by DZone contributors are their own.
Trending

Explainable AI: Making the Black Box Transparent

Building A Log Analytics Solution 10 Times More CostEffective Than Elasticsearch

Playwright JavaScript Tutorial: A Complete Guide

The SPACE Framework for Developer Productivity
Comments