LeetCode "Shortest Word Distance II"

class WordDistance {
unordered_map<string, vector<int>> hm;
public:
WordDistance(vector<string>& words) {
for (int i = 0; i < words.size(); i++)
{
hm[words[i]].push_back(i);
}
}

int shortest(string word1, string word2) {
vector<int> &vec1 = hm[word1];
vector<int> &vec2 = hm[word2];

int ret = INT_MAX;
int i1 = 0, i2 = 0;
while (i1 < vec1.size() && i2 < vec2.size())
{
int v1 = vec1[i1], v2 = vec2[i2];
ret = std::min(ret, abs(v1 - v2));
if (v1 < v2) i1++;
else i2++;
}
return ret;
}
};