Finding paths in a Undirected Graph
In general, you can do a depth-first search or a breadth-first search, although neither one is superior to the other (since it's easy to come up with examples for which one is superior to the other).
Edit: Upon rereading the question and thinking a bit, since you want all paths from C to A, a DFS starting at C would probably make the most sense. Along the way, you'd have to store sequences of edges and throw sequences away if they don't end up at A.
$stdin = fopen('php://stdin','r'); $stdin = fopen('php://stdin','w');
fscanf(STDIN,"%d\n",$n);fscanf(STDIN,"%d\n",$e);$graph = array();for ($i = 0;$i<$n; $i++){ $graph[$i] = array();}for ($i = 0;$i<$e; $i++){ fscanf(STDIN,"%s%s",$x,$y); $row = ord($x[0]) - ord('A'); $row = ord($y[0]) - ord('A'); echo $row." ".$col; $graph[$row][$col] = 1; $graph[$col][$row] = 1;}for ($i = 0;$i <$n; $i++){ for ($j= 0;$j<$n ;$j++) { if ($graph[$i][$j]==1) { echo"1 "; } else{ echo"0 "; } echo "\n"; }}