636. Exclusive Time of Functions

Intuition

The goal is to calculate how much exclusive time each function spends running — i.e., excluding the time consumed by any function it called.

Since the execution happens on a single-threaded CPU, we can simulate the behavior using a stack:

• When a function starts, we pause the currently running function (if any).

• When a function ends, we pop it off the stack and add its time.

• If a function resumes after a nested call, we update its start timestamp accordingly.

Complexity

Space Complexity	Time Complexity
$\text{O}(1)$	$\text{O}(N)$

Code

public int[] exclusiveTime(int totalFunctions, List<String> logs) {
    int[] exclusiveTimes = new int[totalFunctions];

    // Stack to store currently running function info: [functionId, startTime]
    Stack<int[]> callStack = new Stack<>();

    for (String logEntry : logs) {
        String[] parts = logEntry.split(":");
        int functionId = Integer.parseInt(parts[0]);
        String action = parts[1];
        int timestamp = Integer.parseInt(parts[2]);

        if (action.equals("start")) {
            // Pause the current running function, if any
            if (!callStack.isEmpty()) {
                int[] currentFunction = callStack.peek();
                int duration = timestamp - currentFunction[1];
                exclusiveTimes[currentFunction[0]] += duration;
            }

            // Start the new function
            callStack.push(new int[]{functionId, timestamp});

        } else { // action == "end"
            // Function ends, calculate total time it ran (inclusive)
            int[] finishedFunction = callStack.pop();
            int duration = timestamp - finishedFunction[1] + 1;
            exclusiveTimes[finishedFunction[0]] += duration;

            // Resume the parent function (if any), updating its start time
            if (!callStack.isEmpty()) {
                int[] parentFunction = callStack.peek();
                parentFunction[1] = timestamp + 1; // resumes right after current ends
            }
        }
    }

    return exclusiveTimes;
}