An Euler circuit in a directed graph is a cycle in which every edge is visited exactly

Question:

An Euler circuit in a directed graph is a cycle in which every edge is visited exactly once.
a. Prove that a directed graph has an Euler circuit if and only if it is strongly connected and every vertex has equal indegree and outdegree.
b. Give a linear-time algorithm to find an Euler circuit in a directed graph where one exists.