3. Longest Substring Without Repeating Characters，longestrepeating

英文题目

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.

题目大意

给一个字符串，求最长的不重复子串。

解题思路

直接遍历，想到重复，可以用set来保存出现过的字符；

同时，只要和子串的第一个重复就是算是重复的，所以只需要判断第一个就可以。可以建立一个256位大小的整型数，因为ASCII表共能表示256个字符。

java源码

public int lengthOfLongestSubstring(String s) {
    int[] allChar = new int[256];
    int res = 0;
    //子串的起点下标记.
    int left = -1;
    for (int i = 0; i < s.length(); i++) {
        //left 就是遍历之前的没有出现过的字符.
        left = Math.max(left, allChar[s.charAt(i)]);
        //更新出现过的字符位置.
        allChar [s.charAt(i)] = i;
        //遍历到当前字符串,最大的要么之前的最大的长,
         // 要么就是 left到当前字符的步长.
        res = Math.max(res, i - left);
    }
    return res;
}