https://leetcode.com/problems/design-underground-system/

Clarification & Assumption

Description: design an underground railway system that keeps track
             of customer travel times between different stations
functions:
    1. void checkIn(int id, String stationName, int t)
        customer with id check in stationName at time t
    2. void checkOut(int id, String stationName, int t)
        customer with id check out stationName at time t
    3. double getAverageTime(String startStation, String endStation)
        calculate the average time it takes to traverl from start to end stations

https://leetcode.com/problems/factor-combinations/

Clarification & Assumption

Description: Giving a number n, return all possible combinations of its factors

input: int n
output: List<List<Integer>>
for all the possible cases:
    e.g. n = 1, return []
         n = 12, return [[2,6], [3,4], [2,2,3]]

https://leetcode.com/problems/subsets-ii/

Clarification & Assumption:

Description: given an integer array, find all possible subsets
             e.g. [1,2,2] -> [1], [2], [1,2], [2,2], [1,2,2]
Input & Output:
    i. input: String set
    ii. output: List<String>
For all the possible cases:
    there could be duplicate characters

Clarification & Assumption:

Input & Output (signature):
    i. input: int[] array, int k
    ii. output: int[], k smallest in ascending order
For all the possible cases:
    input not sorted
    k >=0, k <= input.length
    array not null
    corner case: array == [], k == 0 → return empty array