76. Minimum Window Substring

class Solution {
    func minWindow(_ s: String, _ t: String) -> String {
        var map = [Character : Int]()   //t's char and count
        var diff = t.count
        var left = 0
        var right = 0
        // var res = ""
        var len = Int.max
        var start = 0
        let sArr = Array(s)
        for char in t {
            map[char, default: 0 ] += 1   // count
        }

        while right < sArr.count {
            let char = sArr[right]

            if let count = map[char] {  //t has this char
                if count > 0 {          //this is useful to diff
                    diff -= 1
                }
                map[char] = count - 1
            }
            if diff == 0 {  // found an ans
                while left <= right{
                    if let count = map[sArr[left]] {
                        if count == 0 {
                            if right - left + 1 < len{
                                len = right - left + 1
                                start = left
                            }
                            map[sArr[left]] = count + 1
                            left += 1
                            diff += 1
                            break
                        }
                        map[sArr[left]] = count + 1
                    }
                    left += 1
                }
            }

            right += 1
        }
        if len == Int.max { return "" }
        //              print(start)
        //              print(len)
        return String(sArr[start ..< start + len])
    }
}