递归在此代码中的工作原理

我试图了解递归的工作原理。有简单的例子，如阶乘等，我清楚地理解它们，但我无法理解这里如何递归分配 halfOne 和 halfTwo。这里的调用顺序是什么，这段代码中发生了什么？感谢任何帮助。提前谢谢你。

#include <iostream>
using namespace std;
const int SIZE = 5;
double average(int nums[], int left, int right, double len);
int main()
{
int nums[SIZE];
int left, right;
double len; //If len was an int, the average would always be zero
double avg = 0;
//These are for testing
long double testNum = 0;
long double testAvg;
//Populate the array with random integers from 1 - 100
for (int i = 0; i < SIZE; i++)
{
nums[i] = (rand() % 100) + 1;
testNum = testNum + nums[i]; //For Testing
}
left = 0;
right = SIZE - 1;
len = right - left + 1; //For computing the average
avg = average(nums, left, right, len);
testAvg = testNum / SIZE; //To test my function
cout << "Average from assignment: " << testAvg << endl;
cout << "Average from function: " << avg << endl;
int x;
cin >> x;
return 0;
}

//This function will find the average of all the numbers in an array
//By summing together every number divided by the total len.
//This works because (1 + 2 + 3)/3 is equal to (1/3) + (2/3) + (3/3)
double average(int nums[], int left, int right, double len)
{
double halfOne, halfTwo;
//This is the base case which will be evaluated
if (left == right)
return (nums[left] / len);
//The array is split until it is of the size 1
//And each element is evaluated with the base case
halfOne = average(nums, left, (left + right) / 2, len);
halfTwo = average(nums, (left + right + 2) / 2, right, len);
//Each half is added together to get the final total average. 
return (halfOne + halfTwo);
}