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.
　

分析：贪心法，从头扫描到尾，O(n)搞定 用一个map[256]来记录出现过的字符位置。遇到没有出现过的字符，将map对应位置标记，并且子串长度+1；遇到出现过的字符，将子串自上一次出现该字符位置前面的截断，对这部分截断的字符，标记map为notFound，重新计算子串长度
参考： http://blog.unieagle.net/2012/09/30/leetcode%E9%A2%98%E7%9B%AE%EF%BC%9Alongest-substring-without-repeating-characters/

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
         int map[256] ;
        
        for(int i = 0; i< 256; i++)
             map[i] = -1;
             
        const int Notfound = -1;    
        int firstPos = 0, lastPos = 0,len = 0;
        int i, j;
        int maxLen = 0;
        
        for(i = 0; i < s.size(); i++ )
        {
        
            int value = map[s[i]] ;
            
            if(value == Notfound)
            {
                len++;
                map[s[i]] = i;
                maxLen = maxLen > len ? maxLen : len;
            }else{
            
                lastPos = value;
                for(j = firstPos ; j < lastPos ; j++)
                        map[s[j]] = Notfound ;
                
                firstPos = lastPos + 1;                
                len = i - firstPos + 1;        
                map[s[i]] = i;                
            }        
        }
        
        return maxLen ;
        
    }
};