CODEWITHSUNDEEP
c#unique
BCL
BCL

Reputation: 1

Find First Unique Number

I recently took a Codility test and the question was to find the first unique number in a numeric sequence. Although I get the correct result using LINQ, it is apparently too expensive computationally and not scalable enough.

How would I improve my solution?

var a = new int[] {1, 2, -3, 4, 5, -6, 0, 8, 9, 1, 2};
const int expected = -3;            
var retVal = -1;
var y = a.GroupBy(z => z).Where(z => z.Count() == 1).Select(z => z.Key).ToList();
if (y.Count > 0) retVal = y[0];

Console.Write(retVal==expected);

Upvotes: 0

Views: 1838

Answers (3)

BCL
BCL

Reputation: 1

I decided to test the various answers posted above and count the number of ticks using the StopWatch class.

Changing y.Count > 0 to y.Any() resulted in the second lowest tick count, but the largest reduction belongs to the group by comment from Baldrick

To get the average each test was run 50 times.

 Ticks
   Min   Max     Avg  Actual
    2  14502     584  -3     Original
    2    919      40  -3     if (y.Any()) actual = y[0];
    2   1423      60  -3     a.GroupBy(g => g).Where(w => w.Count() == 1)
                                            .Select(s => s.Key)
                                            .FirstOrDefault();
    2   1553      65  -3     a.ToLookup(i => i).First(i => i.Count() == 1).Key;
    2    317      15  -3     a.GroupBy(i => i).First(i => i.Count() == 1).Key;

Upvotes: 0

Cadburry
Cadburry

Reputation: 1864

try this

    var result = a.GroupBy(g => g).Where(w => w.Count() == 1).Select(s => s.Key).FirstOrDefault();

Upvotes: 0

Baldrick
Baldrick

Reputation: 11840

How about this:

var result = a.ToLookup(i => i).First(i => i.Count() == 1).Key;

This should give -3.

It builds a Lookup object with a key created using each number in the list, and a value of the same number.

In the case of duplicates, then multiple entries are created under each key. The first unique value will be the first group in the Lookup with one entry.

(You could just as easily use GroupBy, instead of ToLookup. The end result will be the same.)

Upvotes: 1

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