1114. Print in Order
Description
Suppose we have a class:
public class Foo { public void first() { print("first"); } public void second() { print("second"); } public void third() { print("third"); } }
The same instance of Foo
will be passed to three different threads. Thread A will call first()
, thread B will call second()
, and thread C will call third()
. Design a mechanism and modify the program to ensure that second()
is executed after first()
, and third()
is executed after second()
.
Note:
We do not know how the threads will be scheduled in the operating system, even though the numbers in the input seem to imply the ordering. The input format you see is mainly to ensure our tests' comprehensiveness.
Example 1:
Input: nums = [1,2,3] Output: "firstsecondthird" Explanation: There are three threads being fired asynchronously. The input [1,2,3] means thread A calls first(), thread B calls second(), and thread C calls third(). "firstsecondthird" is the correct output.
Example 2:
Input: nums = [1,3,2] Output: "firstsecondthird" Explanation: The input [1,3,2] means thread A calls first(), thread B calls third(), and thread C calls second(). "firstsecondthird" is the correct output.
Constraints:
nums
is a permutation of[1, 2, 3]
.
Solutions
Solution 1: Multithreading + Lock or Semaphore
We can use three semaphores $a$, $b$, and $c$ to control the execution order of the three threads. Initially, the count of semaphore $a$ is $1$, and the counts of $b$ and $c$ are $0$.
When thread $A$ executes the first()
method, it first needs to acquire semaphore $a$. After acquiring successfully, it executes the first()
method, and then releases semaphore $b$. This allows thread $B$ to acquire semaphore $b$ and execute the second()
method.
When thread $B$ executes the second()
method, it first needs to acquire semaphore $b$. After acquiring successfully, it executes the second()
method, and then releases semaphore $c$. This allows thread $C$ to acquire semaphore $c$ and execute the third()
method.
When thread $C$ executes the third()
method, it first needs to acquire semaphore $c$. After acquiring successfully, it executes the third()
method, and then releases semaphore $a$. This allows thread $A$ to acquire semaphore $a$ and execute the first()
method.
The time complexity is $O(1)$, and the space complexity is $O(1)$.
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Solution 2
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