Leetcode 0003: Longest Substring Without Repeating Characters

题目描述：

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

Example 1:

Input: s = “abcabcbb”
Output: 3
Explanation: The answer is “abc”, with the length of 3.

Example 2:

Input: s = “bbbbb”
Output: 1
Explanation: The answer is “b”, with the length of 1.

Example 3:

Input: s = “pwwkew”
Output: 3
Explanation: The answer is “wke”, with the length of 3.
Notice that the answer must be a substring, “pwke” is a subsequence and not a substring.

Example 4:

Input: s = “”
Output: 0

Constraints:

0 <= s.length <= 5 * 104
s consists of English letters, digits, symbols and spaces.

Time complexity: O(n)
同向双指针：
1、指针j 遍历字符，维护一个[i,j]区间始终维持以j结尾的无重复字符的最长子串，用哈希表记录[i,j]区间的每个元素的个数
2、每当j往前走一格时，直接将j位置的元素加入到哈希表中，然后判断j位置的元素在[i,j]区间中是否有重复，若有重复则i往前走一步，并将i位置的元素数量减1，当[i,j]中确保不存在重复元素时，则更新res：比较当前长度 j-i+1 和原长度 res 取最大值。

class Solution {
    
    
    public int lengthOfLongestSubstring(String s) {
    
    
        int res = 0;
        int[] a = new int[256];
        int len = s.length();
        for(int i = 0,j = 0;j<len;j++){
    
    
            int index = s.charAt(j);
            a[index]++;
            while(a[index] > 1){
    
    
                a[s.charAt(i++)]--;
            }
            res = Math.max(res, j-i+1);
        }
        return res;
    }
}