首先談一談在實現中遇到幾個問題和c++不同且容易誤用的點：

1. lua語言中，整型計算可以得出浮點型的結果，可以使用math.floor向下取整，或者math.ceil向上取整

local middle = math.floor((right - left + 1) / 2) + left

2.lua語言中，局部變量不加 local 會被視為全局變量，例如middle如果沒有加 local 聲明，那麽它的值在遞歸調用過程中都是共享的

middle = math.floor((right - left + 1) / 2) + left

完整代碼：

 1 function merge(arr, left, right, middle, result)
 
 2     local resultIndex = left
 3     local arrLeftIndex = left
 4     local arrRightIndex = middle
 5     -- 用臨時的result數組做插入排序
 6     while (arrLeftIndex <= middle - 1 and arrRightIndex <= right) do
 7         if (arr[arrLeftIndex] > arr[arrRightIndex]) then
 8             result[resultIndex] = arr[arrRightIndex]
 
 9             arrRightIndex = arrRightIndex + 1
10         else
11             result[resultIndex] = arr[arrLeftIndex]
12             arrLeftIndex = arrLeftIndex + 1
13         end
14         resultIndex = resultIndex + 1
15     end
16 
17     while (arrLeftIndex <= middle - 1) do
18         result[resultIndex] = arr[arrLeftIndex]
 
19         resultIndex = resultIndex + 1
20         arrLeftIndex = arrLeftIndex + 1
21     end
22 
23     while (arrRightIndex <= right) do
24         result[resultIndex] = arr[arrRightIndex]
25         resultIndex = resultIndex + 1
26         arrRightIndex = arrRightIndex + 1
27     end
28 
29     --排序結果保存到原數組
30     for i = left, right do
31         arr[i] = result[i]
32     end
33 end
34 
35 function mergeSort(arr, left, right)
36     --print(left.." "..right)
37     local result = {}
38     if (1 == right - left) then
39         if (arr[left] > arr[right]) then
40             local temp = arr[left]
41             arr[left] = arr[right]
42             arr[right] = temp
43         end
44     elseif (0 ~= left - right) then
45         local middle = math.floor((right - left + 1) / 2) + left
46         --print(left.." "..right.." "..middle) 分解
47         mergeSort(arr, left, middle - 1, result)
48         mergeSort(arr, middle, right, result)
49         --print(left.." "..right.." "..middle) 合並
50         merge(arr, left, right, middle, result)
51     end
52 end
53 
54 local unsortedArray = {2, 1, 3, 5, 6, 9, 8, 7, 4}
55 
56 mergeSort(unsortedArray, 1, #unsortedArray)
57 
58 for i,v in ipairs(unsortedArray) do
59     io.write(tostring(v).." ")
60 end

lua實現歸並排序