Medium
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.
屬于滑動窗口那種Two Pointers的題, 維持l, r兩個指針，每次遇到沒有出現過的字符，就加入到set里面，并且向右擴展窗口；遇到出現過的字符，就要縮小窗口，從左邊開始刪除，知道刪完這個重復的元素，那么這時候set里面就沒有該元素了，又可以進行新的窗口擴展。
class Solution {
public int lengthOfLongestSubstring(String s) {
if (s == null || s.length() == 0){
return 0;
}
int l = 0;
int r = 0;
int maxLen = 0;
HashSet<Character> set = new HashSet<>();
while (r < s.length() && l < s.length()){
if (!set.contains(s.charAt(r))){
set.add(s.charAt(r));
maxLen = Math.max(maxLen, r - l + 1);
r++;
} else {
set.remove(s.charAt(l));
l++;
}
}
return maxLen;
}
}
