Block swap algorithms swap two elements of an array in computer algorithms. It is simple to swap two non-overlapping regions of an array of equal size. However, it is not simple to swap two non-overlapping regions of an array in-place that are next to each other, but are of unequal sizes. Three algorithms are known to accomplish this: Bentley's Juggling, Gries-Mills, and Reversal. All three algorithms are linear time O(n), (see Time complexity).