While finding Articulation points using DFS, which of the following is a necessary condition for a vertex to be an Articulation point?
1
A leaf of DFS tree can be an articulation point.
2
If u is a vertex in G such that x is an ancestor of u in T and y is a descendent of u in T and all paths from x to y in G must pass through u, then u is an articulation point in T.
3
The root of a DFS tree is an articulation point if and only if it has at least 2 children.
4
A non-root vertex u of a DFS-tree is an articulation point of G if and only if has a child y such that there is no back edge from y or any descendant of y to a proper ancestor of v.