pyUdemyCourse-CyclesTask

# Udemy Course. Foundations of Computer Science: Theory and Practice
# Section 3.5 Cycles.

def cycles(indices):
    # Initialise an empty list, which will become a list of cycles (each cycle being a sub-list)
    all_cycles = []

    for x in indices:       # Work through the given indices list one position at a time
        # Initialise a list for one cycle, containing the index of the start point within indices
        one_cycle = [x]
        while True:         # Keep looping until we break out
            x = indices[x]              # Pick up the index stored at the current position within indices
            if x == one_cycle[0]:       # Have we returned to the start point in the current cycle?
                one_cycle.sort()        # Yes - Sort this one cycle, so that we don't create duplicates
                if one_cycle not in all_cycles:     # Is this a newly discovered cycle?
                    all_cycles.append(one_cycle)    # Yes - append it to our results list
                break                               # And break out to the outer loop to search again

            # We have not hit a cycle point, so append this index to the current cycle
            one_cycle.append(x)

    return all_cycles           # Return our result list of all cycles found


tests = [[2, 0, 1, 4, 3, 5], [0, 1, 2, 3], [3, 2, 0, 1]]
for test in tests:
    print(cycles(test))

# Results:
# [[0, 1, 2], [3, 4], [5]]
# [[0], [1], [2], [3]]
# [[0, 1, 2, 3]]