Find the Smallest Window in String s that Contains?

ll Characters of String t

string MinWindow(string s, string t)
{
if (string.IsNullOrEmpty(s) || string.IsNullOrEmpty(t)) return

"";

Dictionary<char, int> dictT = new Dictionary<char, int>();
foreach (char c in t)
dictT[c] = dictT.ContainsKey(c) ? dictT[c] + 1 : 1;
int required = dictT.Count;
int formed = 0;
Dictionary<char, int> windowCounts = new Dictionary<char,
int>();
int left = 0, right = 0;
int minLen = int.MaxValue, minLeft = 0;

while (right < s.Length)

{
char c = s[right];

windowCounts[c] = windowCounts.ContainsKey(c) ?

windowCounts[c] + 1 : 1;

if (dictT.ContainsKey(c) && windowCounts[c] == dictT[c])
formed++;

while (left <= right && formed == required)

{
if (right - left + 1 < minLen)
{
minLen = right - left + 1;

Follow on:

minLeft = left;
}
char leftChar = s[left];

windowCounts[leftChar]--;

if (dictT.ContainsKey(leftChar) &&

windowCounts[leftChar] < dictT[leftChar])

formed--;

left++;

}

right++;

}
return minLen == int.MaxValue ? "" : s.Substring(minLeft,

minLen);

}

Explanation:

Sliding window with two pointers keeps track of counts of chars matching the target.