Implement strStr() LeetCode Java

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

public class StrInStr {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        String haystack ="1strSTR12str";
        String needle="str";    
        System.out.println(strStr(haystack ,needle));

    }
    public static String strStr(String str,String s) {
        if (str=="") {
            return str;
        }
        char key=s.charAt(0);
        int index=0;
//        int index=str.indexOf(key);
        char[] sch=s.toCharArray();
        char[] strch=str.toCharArray();

        while(index!=-1) {
            index=str.indexOf(key);
            for(int i=0;i<sch.length;i++) {
                if(sch[i]==strch[index+i]) {
                    return s;
                }
            }
            str=str.substring(index+1);
            
        }
        return null;
    }
    





    
    
//方法二    
    public static String strStr2(String haystack, String needle) {
             if (needle.length() == 0)
                return haystack;
           
             for (int i = 0; i < haystack.length(); i++) {
                  if (haystack.length() - i + 1 < needle.length())
                      return null;
           
                  int k = i;
                 int j = 0;
          
                 while (j < needle.length() && k < haystack.length() && needle.charAt(j) == haystack.charAt(k)) {
                     j++;
                     k++;
                     if (j == needle.length())
                         return haystack.substring(i,k);
                 }
          
             }
             return null;
         }
}