如何筛选Qty和为4的MyObject列表项组合（找到即停止）

Hey there, sounds like you need to grab the first set of items from your List(Of MyObject) where the sum of their Qty properties hits your target value (4 in this case), and stop searching once you find that set. Let's work through this.

First, let's make sure we're on the same page with your existing code (I'll add the assumed MyObject class definition too, since it's needed for context):

Public Class MyObject
    Public Property Name As String
    Public Property Qty As Integer
End Class

Dim someValue As Integer = 4
Dim myList As New List(Of MyObject)
myList.Add(New MyObject With {.Name = "N1", .Qty = 3})
myList.Add(New MyObject With {.Name = "N2", .Qty = 1})
myList.Add(New MyObject With {.Name = "N3", .Qty = 2})
myList.Add(New MyObject With {.Name = "N4", .Qty = 1})

Solution: Recursive Backtracking with Early Termination #

Since you only need the first valid combination (not all possible ones), we can use a backtracking approach that stops as soon as it finds a match. This is efficient because we don't waste time checking all possible combinations once we have our result.

Here's the implementation:

Private Function FindFirstCombination(ByVal items As List(Of MyObject), ByVal targetSum As Integer) As List(Of MyObject)
    Dim currentCombo As New List(Of MyObject)
    ' Trigger the backtracking search; return the combo if found
    If BacktrackToFindCombo(items, targetSum, 0, currentCombo) Then
        Return currentCombo
    End If
    ' Return empty list if no valid combo exists
    Return New List(Of MyObject)
End Function

Private Function BacktrackToFindCombo(ByVal items As List(Of MyObject), ByVal remainingSum As Integer, ByVal startIndex As Integer, ByVal currentCombo As List(Of MyObject)) As Boolean
    ' We've hit our target sum - success!
    If remainingSum = 0 Then
        Return True
    End If
    ' No more items to check or we've exceeded the target - exit
    If remainingSum < 0 Or startIndex >= items.Count Then
        Return False
    End If

    ' Try including the current item in the combo
    currentCombo.Add(items(startIndex))
    ' Recurse with the updated remaining sum and next index
    If BacktrackToFindCombo(items, remainingSum - items(startIndex).Qty, startIndex + 1, currentCombo) Then
        Return True
    End If
    ' Backtrack: remove the current item and try skipping it
    currentCombo.RemoveAt(currentCombo.Count - 1)

    ' Try skipping the current item entirely
    Return BacktrackToFindCombo(items, remainingSum, startIndex + 1, currentCombo)
End Function

How to Use It #

Call the function and process your result like this:

Dim resultCombo As List(Of MyObject) = FindFirstCombination(myList, someValue)

' Verify and output the result
If resultCombo.Count > 0 Then
    Console.WriteLine("Found valid combination:")
    For Each item In resultCombo
        Console.WriteLine($"Name: {item.Name}, Qty: {item.Qty}")
    Next
    Console.WriteLine($"Total Qty: {resultCombo.Sum(Function(x) x.Qty)}")
Else
    Console.WriteLine("No combination of items sums to the target value.")
End If

Notes #

• This implementation will return the left-most valid combo first (in your case, N1 and N2, since 3+1=4). If you wanted to prioritize a different type of combo (like the last three items N2+N3+N4 which sum to 4), you could reverse the list before passing it to the function, or adjust the backtracking order. But since you said any valid combo works once found, this default behavior should be fine.
• The backtracking approach avoids duplicate combinations by only checking items from the current startIndex onwards, so you won't get the same items in a different order (like N2 followed by N1) as a separate combo.

内容的提问来源于stack exchange，提问作者Leebeedev