void topologicalSortUtil(int v, boolean visited[], Stack<Integer> stack, ArrayList<Integer>[] adj) {
visited[v] = true;
for (int i : adj[v]) {
if (!visited[i]) {
topologicalSortUtil(i, visited, stack, adj);
}
}
stack.push(v);
}
Here, I am calling an array of arraylist. The error is in the enhanced for loop, as told by my IDE (IntelliJ). It seems like a perfectly valid call as adj[v] would return an ArrayList and the integer i catching each elements of the array as they come and then processing them in the recursive call.
@Alex Shesterov and others, Here is the code using which adj was generated.
ArrayList[] arr = new ArrayList[n+1];
for(int i=0; i<=n; i++){
arr[i] = new ArrayList<Integer>();
}
arr[1].add(Arrays.asList(8,6,2));
arr[2].add(Arrays.asList(7));
arr[3].add(Arrays.asList(6));
arr[4].add(Arrays.asList(5,2));
arr[5].add(Arrays.asList(4,2,3));
arr[6].add(Arrays.asList(3,2,5));
arr[7].add(Arrays.asList(2));
arr[7].add(Arrays.asList(1,2,4));
The code in which you generate the array is incorrect and type-unsafe. You are probably getting warnings about "unsafe types" or "raw types" from the compiler. The following is a use of a raw type:
ArrayList[] arr = new ArrayList[n+1];
You should never use the type ArrayList without some kind of type declaration.
You then initialize each of the elements of arr to be an ArrayList<Integer>. But since this is assigned to a raw typed array element, the compiler doesn't know that the element is an ArrayList<Integer> and doesn't warn you about your next step, which is the bad one:
arr[1].add(Arrays.asList(8,6,2));
You add an element to the list in arr[1]. The add method adds a single element, which is the object that is passed to it. And what is that object? It's a List, which is produced by Arrays.asList(...).
So instead of having an ArrayList<Integer> you have a raw ArrayList which has a single element, and that element is a list of integer. And this, of course, cannot be cast to Integer.
Instead of using add you should have used addAll. The addAll method adds all the elements in the collection it receives to the collection on which it is called.
arr[1].addAll(Arrays.asList(8,6,2));
It would have been better not to mix arrays and lists, but to pass a list of lists and give it the appropriate type.
Try to avoid raw types and eliminate all raw type or unsafe type warnings from your code (not by @SupressWarnings!), unless you start programming libraries for generic collections and you know what you are doing.
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