gistfile1.txt

func lengthOfLongestSubstring(_ s: String) -> Int {
    
    var cache = [Character: Int]()
    var tempMax = 0
    var max = 0
    var start = 0
    
    for c in s.enumerated() {

        // check if we've seen char before
        if cache.contains(where: { $0.0 == c.element }) {
            
            // get its index
            let collisionOffset = cache[c.element]!
            
            // if its withn the current longest string, reset the temp
            if collisionOffset >= start {
                
                tempMax -= collisionOffset - start + 1 // minus one
                start = collisionOffset + 1
                
            } else {
                // else just update the last seen index
                if tempMax > max { max = tempMax }
                cache[c.element] = c.offset
            }
        }
        
        // always increment temp
        tempMax += 1
        if tempMax > max { max = tempMax }
        cache[c.element] = c.offset
        
    }
    
    return max
}