问题描述：
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.
算法分析：使用滑动窗口，设置两个指针，遍历字符串。
public class LongestSubstringWithoutRepeatingCharacters {
public int lengthOfLongestSubstring(String s)
{
int n = s.length();
Set<Character> set = new HashSet<>();
int ans = 0, i = 0, j = 0;//窗口左右指针
while(i < n && j < n)
{
if(!set.contains(s.charAt(j)))
{
set.add(s.charAt(j++));
ans = Math.max(ans, j - i);
}
else//如果有重复元素，则窗口左指针向右移动，直到移动到重复元素的下一位
{
set.remove(s.charAt(i++));
}
}
return ans;
}
}
Longest Substring Without Repeating Characters，求没有重复字符的最长字串
原文：http://www.cnblogs.com/masterlibin/p/5793217.html