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Use these hints if you're stuck. Try solving on your own first.
This problem is equivalent to finding if a cycle exists in a directed graph. If a cycle exists, no topological ordering exists and therefore it will be impossible to take all courses.
<a href="https://www.cs.princeton.edu/~wayne/kleinberg-tardos/pdf/03Graphs.pdf" target="_blank">Topological Sort via DFS</a> - A great tutorial explaining the basic concepts of Topological Sort.
Topological sort could also be done via <a href="http://en.wikipedia.org/wiki/Topological_sorting#Algorithms" target="_blank">BFS</a>.