Created
April 26, 2012 01:52
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Nth smallest number in an unsorted array
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| def nthsmallest arr,startpos,endpos,nth | |
| def choose_pivot arr,startpos,endpos | |
| #THis is for selecting median of three , you can return a random number as well. | |
| if ((arr[startpos] < arr[endpos]) and (arr[startpos] > arr[startpos+ (endpos-startpos)/2])) or ((arr[startpos] > arr[endpos]) and (arr[startpos] < arr[startpos+ (endpos-startpos)/2])) | |
| return startpos | |
| elsif ((arr[endpos] < arr[startpos]) and (arr[endpos] > arr[startpos+ (endpos-startpos)/2])) or ((arr[endpos] > arr[startpos]) and (arr[endpos] < arr[startpos+ (endpos-startpos)/2])) | |
| return endpos | |
| else | |
| return startpos+ (endpos-startpos)/2 | |
| end | |
| end | |
| if (startpos - endpos == 0) and (nth == startpos) | |
| return arr[startpos] | |
| end | |
| pivot = choose_pivot(arr,startpos,endpos) | |
| arr[startpos],arr[pivot] = arr[pivot],arr[startpos] | |
| part_pos = startpos + 1 | |
| for i in startpos+1..endpos | |
| if arr[startpos] > arr[i] | |
| arr[part_pos],arr[i] = arr[i],arr[part_pos] | |
| part_pos += 1 | |
| end | |
| end | |
| arr[startpos],arr[part_pos-1] = arr[part_pos-1],arr[startpos] | |
| if nth < (part_pos-1) | |
| ret = nthsmallest(arr,startpos,part_pos-2,nth) if (part_pos - startpos - 2) >= 0 | |
| elsif nth > (part_pos-1) | |
| ret = nthsmallest(arr,part_pos,endpos,nth) if endpos-part_pos >= 0 | |
| else | |
| ret = arr[part_pos-1] | |
| end | |
| return ret | |
| end |
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