3.无重复字符的最长子串

func lengthOfLongestSubstring(s string) int {
    if s == "" {
        return 0
    }
    max := 0
    for i := 0; i < len(s); i++ {
        start := 0
        num := 0
        lens := len(s)
        hashmap := map[byte]int{}
        for true {
            if hashmap[s[start+i]] == 1 {
                break
            }
            hashmap[s[start+i]] = 1
            start++
            num++
            if start + i == lens {
                break
            }
        }
        if num > max {
            max = num
        }
    }
    return max
}