LeetCode "Minimum Window Substring" - STAR

Main reference: http://zhaohongze.com/wordpress/2014/01/04/leetcode-minimum-window-substring/
The ART of counting. So difficult and beautiful problem. It is all about dynamic window maintanence. Essentially, it is still a fancy linear scanning procedure.

class Solution {
public:
string minWindow(string S, string T) {
if (T.length() > S.length()) return "";

//    Build Dict: Yes chars in T could be duplicated
unordered_map<char, int> dict;
for (int i = 0; i < T.length(); i++)
{
if (dict.find(T[i]) == dict.end())        dict.insert(make_pair(T[i], 1));
else                                    dict[T[i]] ++;
}

//    Build Indices - char - cnt
size_t ttlCnt = 0;
int iLeft = 0;
int minL = 0, minLen = std::numeric_limits<int>::max();
unordered_map<char, size_t> rec;
for (int i = 0; i < S.length(); i++)
{
char c = S[i];
if (dict.find(c) != dict.end())
{
if (rec.find(c) == rec.end())    rec[c] = 1;
else                            rec[c] ++;

if (rec[c] == dict[c]) ttlCnt++;
if (ttlCnt == dict.size())
{
//    Shrink from Left
while ( dict.find(S[iLeft]) == dict.end() ||    // irrelavant char?
(iLeft < i && rec[S[iLeft]] > dict[S[iLeft]])) // redundant char?
{
if (dict.find(S[iLeft]) != dict.end())
rec[S[iLeft]] --;
iLeft++;
}
//    Update record
if ((i - iLeft + 1) < minLen)
{
minL = iLeft;
minLen = (i - iLeft + 1);
}
}
}
}
if (minLen == std::numeric_limits<int>::max()) return "";
return S.substr(minL, minLen);
}
};