Update Exercises Directory Names to Reflect Order

exercises/20_threads/threads1.rs

@@ -0,0 +1,40 @@
+// threads1.rs
+//
+// This program spawns multiple threads that each run for at least 250ms, and
+// each thread returns how much time they took to complete. The program should
+// wait until all the spawned threads have finished and should collect their
+// return values into a vector.
+//
+// Execute `rustlings hint threads1` or use the `hint` watch subcommand for a
+// hint.
+
+// I AM NOT DONE
+
+use std::thread;
+use std::time::{Duration, Instant};
+
+fn main() {
+    let mut handles = vec![];
+    for i in 0..10 {
+        handles.push(thread::spawn(move || {
+            let start = Instant::now();
+            thread::sleep(Duration::from_millis(250));
+            println!("thread {} is complete", i);
+            start.elapsed().as_millis()
+        }));
+    }
+
+    let mut results: Vec<u128> = vec![];
+    for handle in handles {
+        // TODO: a struct is returned from thread::spawn, can you use it?
+    }
+
+    if results.len() != 10 {
+        panic!("Oh no! All the spawned threads did not finish!");
+    }
+
+    println!();
+    for (i, result) in results.into_iter().enumerate() {
+        println!("thread {} took {}ms", i, result);
+    }
+}