#18EasyThread Synchronization· Events and Signalling

Print in Order

One `Foo` object is shared by three threads. Thread A calls `first()`, thread B calls `second()` and thread C calls `third()`. The threads may be started, and may run, in any order. Each method receives a callback that prints its word (`printFirst` prints `first`, and so on). Add synchronisation so that, whatever the scheduling, `second` is printed only after `first`, and `third` only after `second`. Do not rely on sleeps or timing. Busy-waiting on a shared flag is also not acceptable; block the waiting thread with the language's synchronisation primitives. **Test format.** The input `order` is a permutation of `[1, 2, 3]` giving the order in which the judge starts the threads (1 calls `first`, 2 calls `second`, 3 calls `third`). The output is everything printed, concatenated.

Input format

order: permutation of [1, 2, 3], the thread start order

Output format

String: the concatenated output, which must be "firstsecondthird"

Examples

Input: order = [1, 3, 2]

Output: firstsecondthird

Thread 3 starts before thread 2, but it blocks until `second` has printed.

Input: order = [3, 2, 1]

Output: firstsecondthird

Threads start in reverse order; both waiting threads block until their predecessor signals.

Constraints

order is a permutation of [1, 2, 3]
Each method is called exactly once, each from its own thread.

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Operating Systems & Concurrency

#concurrency
#threads
#synchronization
#events