Number of matching pairs

Problem solving with programming: Number of matching pairs
Given an array containing values in the range [-10,10], find out how many number of matching pairs are possible?

A pair of numbers is said to be matching if one value is negative of the other (two zeros are also considered matching pair).

Consider the following examples.
[-6, 0, 1, 6, 0, -1] contains 3 matching pairs. Indices are (0,3), (1,4), (2,5)
[0, 0, 0, -5 ,3, 5] contains 4 matching pairs. Indices are (0,1), (0,2), (1,2), (3,5)
Now, the number of matching pairs is the sum of Count(-x) * Count(x) for 1<=x<=10

The number of possible pairs for n zeros is n*(n-1)/2

cin >> n;


vector<long long> input(21);


int i,j;


for( i = 0; i < n; i++ )


{


int r;


cin >> r;


input[ r+10 ]++;


}


long long result = 0;


i = 0;


j = 20;


while ( i < j )


{


result += input[i]*input[j];


i++;


j--;


}


result += input[10]*(input[10]-1)/2;


cout << result << endl;

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