一. 题目描写叙述

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.

二. 题目分析

题目的大意是。给出一串字符串，找出无反复字符的最长子串，输出其长度。能够使用两个指针，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，记录当前子串长度和最优解的比較结果。然后头指针不断往后扫描。直到扫描到一个字符和尾指针同样，则尾指针继续扫描。当尾指针到达字符串结尾时算法结束。算法复杂度O(n) + O(n) = O(n)。

三. 演示样例代码

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;
    }
};

四. 小结

參考：http://www.cnblogs.com/remlostime/archive/2012/11/12/2766530.html