3. 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.

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        // 定义freq数组用于判断子串中的元素是否重复，freq数组的下标对应ASCII。
        // 每遍历到一个元素?，就会判断freq[?]的值是否为0。
        // 为0则右边界r向前进一步，同时freq[?]++，值变为1。
        // 如果freq[?]的值是1，那么左边界l就向前进，同时freq[l]--变为0，一直遍历到重复的元素?，
        // 将freq[?]--变为0，此时右边界才能继续前进。
        int freq[256] = {0};
        // 滑动窗口[l...r]，初始时[0...-1]滑动窗口不包含任何元素
        int l = 0, r = -1;
        // 最长子串的长度
        int res = 0;

        while (l < s.length()) {
            if (r + 1 < s.length() && freq[s[r + 1]] == 0) {
                freq[s[++r]] ++;
            }
            else {
                freq[s[l++]] --;
            }

            res = max(res, r - l + 1);
        }

        return res;
    }
};