Longest Substring Without Repeating Characters
【題目】
Given a string, find the length of the longest substring without repeating characters.
（給定一個字符串，找字符中的最大非重複子串）
Example 1:
Input: "abcabcbb"
Output: 3
Explanation: The answer is "abc", with the length of 3.
Example 2:
Input: "bbbbb"
Output: 1
Explanation: The answer is "b", with the length of 1.
Example 3:
Input: "pwwkew"
Output: 3
Explanation: 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.
【分析】
首先，這道題目要求是“非重複最長子串”，所以我們應該在第一時間點想到用HashSet容器。
其次，我們用兩個指針，一個遍歷數組，另一個檢測是否有重複的字符。如果發現重複字符，則remove。
最後用Math.max()函數記錄最大的長度即可。
Java實現代碼如下：
class Solution {
public int lengthOfLongestSubstring(String s) {
int res = 0;
if (s == null || s.length() == 0) return res;
//注意區別字符串的length()方法與數組的length屬性
HashSet<Character> set = new HashSet<>();
for (int i = 0, j = 0; i < s.length();) {
if (set.contains(s.charAt(i))) {
set.remove(s.charAt(j++));
} else {
set.add(s.charAt(i++));
res = Math.max(res, set.size());
}
}
return res;
}
}