Explaining a solution

Problems and Contests
#1

could anybody please explain this solution for usaco bronze why did the cow cross the road - II:
#include <bits/stdc++.h>
using namespace std;

int main ()
{
vector vec[26];
string alphabet;
cin >> alphabet;

int ans = 0;

for (int i = 0; i<52; i++)
{
    int cur = alphabet[i] - 'A';
    vec[cur].push_back(i);
}

for (int i = 0; i<26; i++)
{
    for (int j = i+1; j<26; j++)
    {
        if (vec[i][0] < vec[j][0] && vec[i][1] > vec[j][0] && vec[i][1] < vec[j][1] || vec[i][0] > vec[j][0] && vec[j][1] > vec[i][0] && vec[i][1] > vec[j][1])
        {
            ans++;
        }
    }
}

cout << ans << '\n';

}