[LeetCode] Longest Substring Without Repeating Characters

Given a string, find the length of the longest substring without repeating characters. For example, the longest substring without repeating letters for "abcabcbb" is "abc", which the length is 3. For "bbbbb" the longest substring is "b", with the length of 1.

用两个指针，一个指向当前子串的头，一个指向尾，尾指针不断往后扫描，当有字符前面出现过了，记录当前子串长度和最优解的比较结果。然后头指针不断往后扫描，直到扫描到一个字符和尾指针相同，则尾指针继续扫描，当尾指针到达字符串结尾，算法结束。复杂度O(n) + O(n) = O(n)

class Solution {
private:
    bool canUse[256];
public:
    int lengthOfLongestSubstring(string s) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        memset(canUse, true, sizeof(canUse));
        
        int count = 0;
        int start = 0;
        int ret = 0;
        for(int i = 0; i < s.size(); i++)
        {
            if (canUse[s[i]])
            {
                canUse[s[i]] = false;
                count++;
            }
            else
            {
                ret = max(ret, count);
                while(true)
                {
                    canUse[s[start]] = true;
                    count--;
                    if (s[start] == s[i])
                        break;
                    start++;
                }
                start++;
                canUse[s[i]] = false;
                count++;
            }
        }
        
        ret = max(ret, count);
        
        return ret;
    }
};