Topological sort using DFS without recursion

Question

I know the common way to do a topological sort is using DFS with recursion. But how would you do it using stack instead of recursion? I need to obtai

Answer 1

I think your code is a good non-recursive DFS . The key point of topological sort is that:

When you pick a node to push into the stack. This node must have no precessor( has a in-degree of 0). This means you are doing a DFS base on in-degree in stead of push all the adjacent nodes into the stack, you always pick the one with 0 in-degree

So you push every node that have no precessor in the stack. Pop one, print it and remove it from all its adjacent nodes' precessor list ( or decrease its adjacent nodes' in-degree by 1). This need you to edit your adjacent list. Than push all its adjacent nodes that has in-degree of 0 now in to the stack( this phase may fail but no problem, you still got some nodes in the stack). Then pop the next one. Repeat until the stack is empty. The node sequence that you printed is the topological sort result of the graph.