Exclusive Time of Functions

Problem

Given the running logs of n functions that are executed in a nonpreemptive single threaded CPU, find the exclusive time of these functions.

Each function has a unique id, start from 0 to n-1. A function may be called recursively or by another function.

A log is a string has this format : function_id:start_or_end:timestamp. For example, "0:start:0" means function 0 starts from the very beginning of time 0. "0:end:0" means function 0 ends to the very end of time 0.

Exclusive time of a function is defined as the time spent within this function, the time spent by calling other functions should not be considered as this function's exclusive time. You should return the exclusive time of each function sorted by their function id.

Example 1:
Input:
n = 2
logs = 
["0:start:0",
 "1:start:2",
 "1:end:5",
 "0:end:6"]
Output:[3, 4]
Explanation:
Function 0 starts at time 0, then it executes 2 units of time and reaches the end of time 1. 
Now function 0 calls function 1, function 1 starts at time 2, executes 4 units of time and end at time 5.
Function 0 is running again at time 6, and also end at the time 6, thus executes 1 unit of time. 
So function 0 totally execute 2 + 1 = 3 units of time, and function 1 totally execute 4 units of time.

Note: Input logs will be sorted by timestamp, NOT log id. Your output should be sorted by function id, which means the 0th element of your output corresponds to the exclusive time of function 0. Two functions won't start or end at the same time. Functions could be called recursively, and will always end. 1 <= n <= 100

Solution

Stack Solution

public class Solution {
    public int[] exclusiveTime(int n, List<String> logs) {
        int[] res = new int[n];
        Stack<Integer> s = new Stack<>();
        int pre = 0;
        for (String log : logs) {
            String[] strs = log.split(":");
            if (strs[1].equals("start")) {
                if (!s.isEmpty()) res[s.peek()] += Integer.parseInt(strs[2]) - pre;
                s.push(Integer.parseInt(strs[0]));
                pre = Integer.parseInt(strs[2]);
            }
            else {
                res[s.pop()] += Integer.parseInt(strs[2]) - pre + 1;
                pre = Integer.parseInt(strs[2]) + 1;
            }
        }
        return res;
    }
}

Analysis

Each method is called by another being executed method
Hence we use Stack<Integer> to those methods with their ids (strs[0])
We also have a pre to indicate the starting time of previous executed method
If a new method is called, we use stack.peek() to get previous method
And we update it's time by subtracting previous starting time (pre) from current starting time
Then we add this newly called method into our stack and update the pre
When a method ends, we use stack.pop() to get the updated index of res
And this time we get the length with + 1 because we know pre is the starting time of current method and the method ends at the very ending of its indicating end time strs[2]