# leetcode_OJ WC146 Report and some notes

These days I usually write the solution report on leetcode official website more than my own github website, serving as a path to communicate with pro algorithmic coders and exchange the ideas throughout the world.

## PA. 1128. Number of Equivalent Domino Pairs

• Approach: Use pair and let the smaller value always at the front, then C(i, 2) for combinations of 2 in i same dominoes group
• Analysis:
• Time complexity: O(N)
• Space complexity: O(N) for storing the results
class Solution {
public:
int numEquivDominoPairs(vector<vector<int>>& d)
{
int res = 0, n = d.size();
map<pair<int, int>, int> m;

for(int i = 0; i < n; i++)
{
if(d[i][0] > d[i][1])
{
m[make_pair(d[i][1], d[i][0])]++;
}
else
{
m[make_pair(d[i][0], d[i][1])]++;
}
}
auto it = m.begin();
while(it != m.end())
{
if(it -> second > 1)
{
res += (it -> second) * (it -> second - 1) / 2 ;
}
it++;
}
return res;
}
};