All algorithms have a best, worst and average case. Algorithms that always perform in constant time have a best, worst and average of O(1).
There is no worst case for merge sort. Each sort takes the same amount of steps, so the worst case is equal to the average case and best case. In each case it has a complexity of O( N * log(N) ).
Can't say without some detail about the algorithm in question.
Merge sort (or mergesort) is an algorithm. Algorithms do not have running times since running times are determined by the algorithm's performance/complexity, the programming language used to implement the algorithm and the hardware the implementation is executed upon. When we speak of algorithm running times we are actually referring to the algorithm's performance/complexity, which is typically notated using Big O notation. Mergesort has a worst, best and average case performance of O(n log n). The natural variant which exploits already-sorted runs has a best case performance of O(n). The worst case space complexity is O(n) auxiliary.
Merge sort is O(n log n) for both best case and average case scenarios.
The best and worst case time complexity for heapsort is O(n log n).
There is no worst case for merge sort. Each sort takes the same amount of steps, so the worst case is equal to the average case and best case. In each case it has a complexity of O( N * log(N) ).
The memory complexity of the quicksort algorithm is O(log n) in the best and average cases, and O(n) in the worst case.
The space complexity of the quicksort algorithm is O(log n) in the best and average cases, and O(n) in the worst case.
These are terms given to the various scenarios which can be encountered by an algorithm. The best case scenario for an algorithm is the arrangement of data for which this algorithm performs best. Take a binary search for example. The best case scenario for this search is that the target value is at the very center of the data you're searching. So the best case time complexity for this would be O(1). The worst case scenario, on the other hand, describes the absolute worst set of input for a given algorithm. Let's look at a quicksort, which can perform terribly if you always choose the smallest or largest element of a sublist for the pivot value. This will cause quicksort to degenerate to O(n2). Discounting the best and worst cases, we usually want to look at the average performance of an algorithm. These are the cases for which the algorithm performs "normally."
The space complexity of the quick sort algorithm is O(log n) in the best and average cases, and O(n) in the worst case.
The space complexity of the Quick Sort algorithm is O(log n) in the best and average cases, and O(n) in the worst case.
The time complexity of the quicksort algorithm is O(n log n) in the average case and O(n2) in the worst case.
The time complexity of the quick sort algorithm is O(n log n) in the average case and O(n2) in the worst case.
The memory complexity of the quick sort algorithm is O(log n) in the best case and O(n) in the worst case.
Asymptotic
The time complexity of the Quick Sort algorithm is O(n log n) on average and O(n2) in the worst case scenario. The space complexity is O(log n) on average and O(n) in the worst case scenario.
The best-case time complexity of the Bubble Sort algorithm is O(n), where n is the number of elements in the array. This occurs when the array is already sorted. The worst-case time complexity is O(n2), which happens when the array is sorted in reverse order.