使用linq避免嵌套循环

Question

I want to create a method to find the index of two numbers whose sum is target .

So, I have created this method:

public static int[] TwoSum(int[] nums, int target)
{
    for (var i = 0; i < nums.Length; i++)
    {
        for (var j = 0; j < nums.Length; j++)
        {
            if (j == i) continue;

            if (nums[i] + nums[j] == target)
            {
                return new[] { i, j };
            }
        }
    }
}

Which works fine. However, I am trying to learn myself some LINQ and can't figure it out. I looked at various examples, but I always end up stuck because I am using the same array twice. So I don't know what to select and how to access it twice while also making sure it doesn't go through the same indexes twice.

Any help getting a LINQ from the above loops would be appreciated.

Sample data:

var nums = new [] { 2, 7, 11, 15 };
var target = 9;

Answer 1

The LINQ equivalent of your loop would be:

from x in nums.Select((n, i) => new { Number = n, Index = i })
from y in nums.Select((n, i) => new { Number = n, Index = i })
where x.Index != y.Index && x.Number + y.Number == target
select new [] { x.Index, y.Index }

But this requires the creation of anonymous type instance so I would say the loop version is much better and efficient.

Answer 2

Here it is as a single LINQ statement, using SelectMany . With LINQ, you have to flip your mind upside down... just type return and then go from there.

public static int[] TwoSumWithLinq(int[] nums, int target)
{
    return nums.SelectMany
    (
        (m, i) => nums.Select
            (
                (n,j) => new { n, m, i, j }
            )
    )
    .Where
    (
        r => (r.i != r.j) && (r.m + r.n == target)
    )
    .Select
    (
        r => new int[] { r.i, r.j }
    )
    .FirstOrDefault();
}

Link to working example on DotNetFiddle

Answer 3

I would go with the slightly cleaner version:

public static int[] TwoSum(int[] nums, int target)
{
    return
    (
        from i in Enumerable.Range(0, nums.Length)
        from j in Enumerable.Range(0, nums.Length)
        where i != j && nums[i] + nums[j] == target
        select new [] { i, j }
    ).FirstOrDefault();
 }

Answer 4

public static int[] TwoSum(int[] nums, int target)
{
    int[] indexes = Enumerable.Range(0, nums.Length).ToArray();

    var result =  from i in indexes
                  from j in indexes
                  where i != j && (nums[i] + nums[j]) == target
                  select new []
                  {
                      i,
                      j
                  };

    return result.FirstOrDefault();
}

Please note that the method will return null if there aren't any members in the array that will match the condition (sum of members = target)