[LeetCode] Longest substring without repeated characters

这题的idea就是从前往后扫，用个hashtable（其实是数组）把上次出现过的位置记下来，遇到出现过的，就从上次出现过的地方开始往下算。 具体实现细节对于新手还是有些坑，比如我刚开始就觉得从新的start开始扫的话，要把hashtable重新initialize，但实际上加一个条件就能避免。另外最长substring出现在最后也有点费思量。还有hashtable要initialize 为-1等也需要注意。 

public class Solution {
   public int lengthOfLongestSubstring(String s) {
      int start = 0, max = 0;
      int [] last = new int[256];  
      Arrays.fill(last, -1); //initialize as -1, since 0 is also a valid position. 

      for (int i = 0; i < s.length(); i++){
         int c = s.charAt(i);
         if (last[c] >= start){  //with this check, you don't need to count the appreance again. 
            max = Math.max(max, i - start);
            start = last[c] + 1;
         }
         last[c] = i;
      }
      max = Math.max(max, s.length() - start);  //don't forget the last character, like "abcd"
      return max;
   }

}