LeetCode----28. Implement strStr()

Implement strStr().

Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

public class Solution {
public int strStr(String haystack, String needle) {
if(haystack==null||needle==null||needle.length()<0||haystack.length()<0||needle.length()>haystack.length()){return -1;}
for(int i=0;i<haystack.length()-needle.length()+1;i++){
int j=0;
for(j=0;j<needle.length();j++){
if(haystack.charAt(i+j)!=needle.charAt(j)){
break;
}
}
if(j==needle.length()){return i;}
}
return -1;
}
}