LeetCOde------3.Longest Substring Without Repeating Characters

Given a string, find the length of the longest substring without repeating characters.
Examples:
Given 

"abcabcbb"

, the answer is 

"abc"

,
which the length is 3.
Given 

"bbbbb"

, the answer is 

"b"

,
with the length of 1.
Given 

"pwwkew"

, the answer is 

"wke"

,
with the length of 3. Note that the answer must be a substring, 

"pwke"

 is
a subsequence and not a substring.

public class Solution{
public static int lengthOfLongestSubstring(String s) {
if(s==null)
return 0;
char[] arr = s.toCharArray();
int pre = 0;

HashMap<Character, Integer> map = new HashMap<Character, Integer>();

for (int i = 0; i < arr.length; i++) {
if (!map.containsKey(arr[i])) {
map.put(arr[i], i);
} else {
pre = Math.max(pre, map.size());
i = map.get(arr[i]);
map.clear();
}
}

return Math.max(pre, map.size());
}
}