What is in-place sorting?
An in-place sorting algorithm uses constant extra space even for producing the output (modifies the given array only). For example, Insertion Sort and Selection Sorts are in-place sorting algorithms and a typical implementation of Merge Sort is not in-place.
What are Internal and External Sortings?
When all data that needs to be sorted cannot be placed in-memory at a time, the sorting is called external sorting. External Sorting is used for massive amount of data. Merge Sort and its variations are typically used for external sorting. Some extrenal storage like hard-disk, CD, etc is used for external storage.
When all data is placed in-memory, then sorting is called internal sorting.
What is stable sorting?
See Stable Sorting Algorithms
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