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Cycle sort is an in-place, unstable sorting algorithm, a comparison sort that is theoretically optimal in terms of the total number of writes to the original array, unlike any other in-place sorting algorithm. It is based on the idea that the permutation to be sorted can be factored into cycles, which can individually be rotated to give a sorted result. Unlike nearly every other sort, items are ''never'' written elsewhere in the array simply to push them out of the way of the action. Each value is either written zero times, if it's already in its correct position, or written one time to its correct position. This matches the minimal number of overwrites required for a completed in-place sort. Minimizing the number of writes is useful when making writes to some huge data set is very expensive, such as with EEPROMs like Flash memory where each write reduces the lifespan of the memory. == Algorithm == The following algorithm finds cycles and rotates them, giving a sorted result. Arrays are zero-indexed. # Sort an array in place and return the number of writes. procedure cycleSort(array): writes = 0 # Loop through the array to find cycles to rotate. for cycleStart from 0 to length(array) - 2, inclusive: item = array() # Find where to put the item. pos = cycleStart for i from cycleStart + 1 to length(array) - 1, inclusive: if array() < item: pos += 1 # If the item is already there, this is not a cycle. if pos == cycleStart: continue # Otherwise, put the item there or right after any duplicates. while item == array(): pos += 1 array(), item = item, array() writes += 1 # Rotate the rest of the cycle. while pos != cycleStart: # Find where to put the item. pos = cycleStart for i from cycleStart + 1 to length(array) - 1, inclusive if array() < item: pos += 1 # Put the item there or right after any duplicates. while item == array(): pos += 1 array(), item = item, array() writes += 1 return writes 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Cycle sort」の詳細全文を読む スポンサード リンク
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