[LeetCode] Longest substring without repeated characters

這題的idea就是從前往後掃，用個hashtable（其實是數組）把上次出現過的位置記下來，遇到出現過的，就從上次出現過的地方開始往下算。 具體實現細節對於新手還是有些坑，比如我剛開始就覺得從新的start開始掃的話，要把hashtable重新initialize，但實際上加一個條件就能避免。另外最長substring出現在最後也有點費思量。還有hashtable要initialize 爲-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;
   }

}