Implement strStr() LeetCode Java

Posted 2020-11-17 昵称真难想

描述
Implement strStr().
Returns a pointer to the first occurrence of needle in haystack, or null if needle is not part of haystack.
分析
暴力算法的复杂度是 O(m ? n)，代码如下。更高效的的算法有 KMP 算法、Boyer-Mooer 算法和
Rabin-Karp 算法。面试中暴力算法足够了，一定要写得没有 BUG。

needle是不是haystack的子串，是的话就返回这个子串

代码

 1 public class StrInStr {
 2 
 3     public static void main(String[] args) {
 4         // TODO Auto-generated method stub
 5         String haystack ="1strSTR12str";
 6         String needle="str";    
 7         System.out.println(strStr(haystack ,needle));
 8 
 9     }
10     public static String strStr(String str,String s) {
11         if (str=="") {
12             return str;
13         }
14         char key=s.charAt(0);
15         int index=0;
16 //        int index=str.indexOf(key);
17         char[] sch=s.toCharArray();
18         char[] strch=str.toCharArray();
19 
20         while(index!=-1) {
21             index=str.indexOf(key);
22             for(int i=0;i<sch.length;i++) {
23                 if(sch[i]==strch[index+i]) {
24                     return s;
25                 }
26             }
27             str=str.substring(index+1);
28             
29         }
30         return null;
31     }
32     





33     
34     
35 //方法二    
36     public static String strStr2(String haystack, String needle) {
37              if (needle.length() == 0)
38                 return haystack;
39            
40              for (int i = 0; i < haystack.length(); i++) {
41                   if (haystack.length() - i + 1 < needle.length())
42                       return null;
43            
44                   int k = i;
45                  int j = 0;
46           
47                  while (j < needle.length() && k < haystack.length() && needle.charAt(j) == haystack.charAt(k)) {
48                      j++;
49                      k++;
50                      if (j == needle.length())
51                          return haystack.substring(i,k);
52                  }
53           
54              }
55              return null;
56          }
57 }