Trees are somewhat less complicated than the graphs, which makes things like data searching easier, when the data has the structure of tree. But, not all data can be represented by the tree. Explain an example of a data set which can't be represented by the tree, but that be represented by more general graph. Discuss why it can't be represented by a tree.

The set of all possible sequences of moves in chess game can be represented by tree (decision tree). If you were to write down a chess-playing computer program which can find out the best move at each step by searching the tree of possible moves and outcomes, would you use the depth-first or a breadth-first search for best move at each step in the game? Explain.