计算给定总和的对

给定一个整数数组和一个数字"sum"，找到数组中总和等于"sum"的整数对的数量。这是Geeks for Geeks的解决方案：

// C++ implementation of simple method to find count of 
// pairs with given sum. 
#include <bits/stdc++.h> 
using namespace std; 
// Returns number of pairs in arr[0..n-1] with sum equal 
// to 'sum' 
int getPairsCount(int arr[], int n, int sum) 
{ 
unordered_map<int, int> m; 
// Store counts of all elements in map m 
for (int i=0; i<n; i++) 
m[arr[i]]++; 
int twice_count = 0; 
// iterate through each element and increment the 
// count (Notice that every pair is counted twice) 
for (int i=0; i<n; i++) 
{ 
twice_count += m[sum-arr[i]]; 
// if (arr[i], arr[i]) pair satisfies the condition, 
// then we need to ensure that the count is 
// decreased by one such that the (arr[i], arr[i]) 
// pair is not considered 
if (sum-arr[i] == arr[i]) 
twice_count--; 
} 
// return the half of twice_count 
return twice_count/2; 
} 
// Driver function to test the above function 
int main() 
{ 
int arr[] = {1, 5, 7, -1, 5} ; 
int n = sizeof(arr)/sizeof(arr[0]); 
int sum = 6; 
cout << "Count of pairs is " 
<< getPairsCount(arr, n, sum); 
return 0; 
} 

我最大的问题是，什么是总和？它没有声明，因此必须以C++构建。但是，我找不到它的任何文档，也不确定它是如何工作的。 我正在尝试遵循代码，并且 sum-arr 值没有意义。

到达 [1,5,8，-1,5]

m [0,1,0,0,0]

总和 [5,1，-2,7,1]