Minimum Window Substring - Implementation

Here is the implementation:

function minWindow(s, t):
    required = frequency map of t
    have = 0, need = required.size
    window = empty map
    minLen = infinity, result = ""
    left = 0

    for right from 0 to s.length - 1:
        c = s[right]
        window[c] += 1

        if c in required and window[c] == required[c]:
            have += 1

        while have == need:
            if right - left + 1 < minLen:
                minLen = right - left + 1
                result = s[left..right]

            window[s[left]] -= 1
            if s[left] in required and window[s[left]] < required[s[left]]:
                have -= 1
            left += 1

    return result

$O(|s| + |t|)$ time, $O(|s| + |t|)$ space.