双指针

func lengthOfLongestSubstring(s string) int {
    if len(s) == 0{
        return 0
    }
    var freq [256]int
    result, left, right := 0, 0, -1
    for left < len(s) {
        var leftpos = s[left] - 'a'
        if right + 1 < len(s) && freq[s[right + 1] - 'a'] == 0 {
            freq[s[right + 1] - 'a'] ++
            right ++
        } else {
            freq[leftpos] --
            left ++
        }
        result = max(result, right - left + 1)
    }
    return result
}

func max(a int, b int) int {
    if a > b {
        return a
    }
    return b
}