Leetcode 3[medium]--- Longest Substring Without Repeating Characters
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.
思路：题意简单，找到给定的string中没有重复字符的最长的亚string。
用字典的key和value记录下每个字符对应的位置，后出现的重复字符的位置会覆盖掉之前出现的。start记录下相同字符的上一次出现位置+1