No.003 Longest Substring Without Repeating Characters

 1 public class Num3 {
 2     /*
 3      * 方法一：暴力搜索，复杂度 O（n^3）
 4      */
 5     public int lengthOfLongestSubstring(String s) {
 6         if(s == null || s.length() == 0){
 7             return 0 ;
 8         }
 9         String sub ;
10         for(int subLen = s.length() ; subLen > 0 ; subLen--){
11             for(int startIndex = 0 ; startIndex <= (s.length()-subLen) ; startIndex++){
12                 //列出所有子串，然后判断子串是否满足有重复
13                 if(startIndex != (s.length()-subLen)){
14                     sub = s.substring(startIndex, startIndex+subLen) ;
15                 }else{
16                     sub = s.substring(startIndex) ;
17                 }
18                 if(!isRepeat(sub)){
19                     return subLen ;
20                 }
21             }
22         }
23         
24         return 1 ;
25     }
26     
27     private boolean isRepeat(String s){
28         for(int i = 1 ; i < s.length(); i++){
29             if(s.substring(i).contains(s.substring(i-1, i))){
30                 return true ;
31             } 
32         }
33         return false ;
34     }
35     
36     /*
37      * 方法二：用hash的方法加上动态规划求解
38      */
39     public int lengthOfLongestSubstring2(String s) {
40         if(s == null || s.length() == 0){
41             return 0 ;
42         }
43         int cur_len = 1 ;      //lenght of current substring
44         int max_len = 1 ;
45         int prev_index ;      //  previous index
46         int [] visited = new int [256] ;
47         char [] arr = s.toCharArray() ;
48         /* Initialize the visited array as -1, -1 is used to
49            indicate that character has not been visited yet. */
50         for(int i = 0 ; i < 256 ; i++){
51             visited[i] = -1 ;
52         }
53         /* Mark first character as visited by storing the index
54            of first   character in visited array. */
55         visited[arr[0]] = 0 ;
56         
57         /* Start from the second character. First character is
58            already processed (cur_len and max_len are initialized
59            as 1, and visited[arr[0]] is set */
60         for(int i = 1 ; i < arr.length ; i++){
61             prev_index = visited[arr[i]] ;
62             
63             /* If the current character is not present in the
64                already processed substring or it is not part of
65                the current NRCS, then do cur_len++ */
66             if(prev_index == -1 || i - cur_len > prev_index){
67                 cur_len++ ;
68             }else{
69                 /* Also, when we are changing the NRCS, we
70                    should also check whether length of the
71                    previous NRCS was greater than max_len or
72                    not.*/
73                 if(cur_len > max_len){
74                     max_len = cur_len ;
75                 }
76                 // update the index of current character
77                 cur_len = i - prev_index ;    
78             }
79             
80             visited[arr[i]] = i ;
81         }
82         
83         // Compare the length of last NRCS with max_len and
84         // update max_len if needed
85         if (cur_len > max_len){
86             max_len = cur_len ;
87         }
88         
89         return max_len ;
90         
91     }
92 
93 }