 Hello everyone, this is Alice Gao. In this video, I'm going to trace breadth-first search on a search graph. This is the search graph. I'm going to add nodes to the frontier in alphabetical order. During this process, I will keep track of the frontier and the search tree. Let's get started. Let's add the initial state S to the frontier and to the search tree. Step one, the oldest node added to the frontier was S. S is the first node expanded. Remove S from the frontier. S is not a goal, let's expand S. S has three successors, D, E, and P. Let's add them to the frontier in alphabetical order and also add them to the search tree. Step two, the oldest node added to the frontier was D. D is the second node expanded. Remove D from the frontier. D is not a goal, let's expand D. D has three successors, B, C, and E. Let's add them to the frontier in alphabetical order and add them to the search tree. Step three, the oldest node added to the frontier was E. E is the third node expanded. Remove E from the frontier. E is not a goal, let's expand E. E has two successors, H and R. Let's add them to the frontier in alphabetical order and add them to the search tree. Step four, the oldest node added to the frontier was P. P is the fourth node expanded. Remove P from the frontier. P is not a goal, let's expand P. P has one successor Q. Add Q to the frontier and to the search tree. So far, BFS has explored the first two levels and has not found a goal yet. I will skip the rest of the steps and show you the final search tree. Let me make some observations. First, BFS explores the search tree level by level until it finds a goal. Second, the order of adding nodes to the frontier determines the order of exploring the nodes in each level. Since we added nodes to the frontier in alphabetical order, BFS explores each level in alphabetical order as well. As a practice question, try tracing BFS on the search graph by adding nodes to the frontier in reverse alphabetical order. That's everything for this example on tracing BFS on a search graph. Thank you very much for watching. I will see you in the next video. Bye for now.