LEETCODE 636: CPU EXCLUSIVE TIME: STACK PATTERN:Understanding Stack-Based Execution Time Calculation

Welcome to our comprehensive walkthrough of LeetCode Problem 636: Exclusive Time of Functions! In this video, we will delve deep into this intriguing problem, which focuses on calculating the exclusive time of functions in a multi-threaded environment. This problem is an excellent opportunity to understand stack-based execution time calculations, a vital concept in computer science and software engineering.
What You Will Learn:
Understanding the Problem Statement:
The problem requires us to determine the exclusive time spent by each function during its execution. The exclusive time of a function is the total time the function spends executing, excluding the time spent in any nested function calls.
We are provided with a list of logs, each representing the start or end of a function call with a specific timestamp. Our task is to process these logs to calculate the exclusive time for each function.
Analyzing the Input and Output:
The input consists of an integer n representing the number of functions and a list of logs. Each log is a string in the format "function_id:start_or_end:timestamp".
The output should be a list where the i-th element is the exclusive time of the i-th function.
Concepts and Data Structures:
To solve this problem, we need to use a stack data structure. The stack will help us keep track of the function calls as we encounter their start and end logs.
We will also maintain an array to store the exclusive time for each function.
Step-by-Step Solution Approach:
Initialization: Start by initializing a stack to keep track of the function calls and an array to store the exclusive times.
Processing Logs: Iterate through the logs. For each log, determine if it is a start or end log.
If it is a start log, push the function id and timestamp onto the stack.
If it is an end log, pop the top element from the stack. The difference between the current timestamp and the popped timestamp will give the duration of the function execution.
Exclusive Time Calculation: Update the exclusive time for the current function. If there are nested function calls, ensure to subtract the nested time from the current function's exclusive time.
Handling Overlapping Functions: Take care to handle overlapping function calls by correctly updating the timestamps and managing the stack.
Edge Cases and Considerations:
Ensure that the input logs are processed in order and handle any potential discrepancies in the log format.
Consider the scenarios where functions start and end at the same timestamp and how to manage such cases.
Optimization and Efficiency:
The solution should efficiently process the logs in a single pass, ensuring a time complexity of O(m), where m is the number of logs.
Space complexity should be O(n) for storing the stack and the exclusive time array.
Example Walkthrough:
Let's walk through a detailed example to solidify our understanding of the solution approach:
Example:
Input:
n = 2
logs = ["0:start:0", "1:start:2", "1:end:5", "0:end:6"]
Output:
[3, 4]
Explanation:
Function 0 starts at timestamp 0.
Function 1 starts at timestamp 2.
Function 1 ends at timestamp 5.
Function 0 ends at timestamp 6.
Exclusive Time Calculation:
Function 0's exclusive time is 6 - 0 + 1 - (5 - 2 + 1) = 3.
Function 1's exclusive time is 5 - 2 + 1 = 4.
Practical Applications:
Understanding the exclusive time of functions is not just a theoretical exercise. It has practical applications in performance profiling and optimization. When developing multi-threaded applications, it's crucial to measure the time each function spends executing to identify performance bottlenecks and optimize resource utilization.
Additional Tips:
Debugging: When implementing the solution, use print statements or a debugger to trace the function calls and stack operations. This can help identify any issues with the logic.
Testing: Test your solution with various test cases, including edge cases such as overlapping function calls, functions that start and end at the same timestamp, and large inputs to ensure scalability.
Code Readability: Ensure your code is clean and well-documented. Use meaningful variable names and add comments to explain the logic, especially in complex sections of the code.
Conclusion:
In this video, we've thoroughly explored LeetCode Problem 636: Exclusive Time of Functions. We covered the problem statement, analyzed the input and output, and discussed the solution approach using stack-based execution time calculation. We also walked through a detailed example to understand the process and provided practical tips for debugging and testing.
By mastering this problem, you gain a deeper understanding of function call management in multi-threaded environments, a skill that is highly valuable in software development and performance optimization.
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