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topologival sort is ancestor sort, means parent should also apprea before the their child in a sorting sequence.

First about DFS: It can be InOrder, PreOrder or PostOrder. How to modify it to BFS?
well the question is not quite clear here. Should the BFS be implemented using DFS?
BFS can be implemented using queues.
Second: Topological Sort outputs those vertices with indegrees of 0 first, removing them, find the remaining vertices of indegree 0 until the whole graph is exhausted. (Well indegree means the number of incoming edges on a vertex in a graph). This also can be implemented using queues.

Modification of a DFS implementation to BFS :- simply replace the stack in the implementation with a Queue.

depth(Tree t)
{
if(t==null)
return 0;
return max(1+depth(t->left),1+depth(t->right))


}