Solution:

Leetcode 003: Longest Substring Without Repeating Characters

Idea:

For this question, we create an array to record the previous occurrence index of certain character. For example, a[(int)'A'] represents the last occurrence index of 'A'. Also, tow pointers, beg and end, is needed. beg points to where we start our substring, and end points to the end of the longest substring starting from beg.
We continuously move end pointer back until we encounter i such that beg <= a[(int)s.charAt(i)], which means there will be repeating characters (s.charAt(a[(int)s.charAt(i)]) and s.charAt(i) should be the same) if we add current character to the substring. At this time, we calculate the lenght current substring and compare to the max.

Extra:

After finishing the scanning, we need to check again the lenght of substring from beg to end, comparing with the max length, before return the result.