Implement strStr()

public String strStr(String haystack, String needle) {

int lenH = haystack.length();
int lenN = needle.length();
if(lenH < lenN ){
return null;
}

for(int i = 0; i < lenH - lenN + 1; i ++){

int j = 0;
while(j < lenN){
if(haystack.charAt(i + j) != needle.charAt(j)){
break;
}
j ++;
}
if(j == lenN){
return haystack.substring(i, lenH);
}
}
return null;
}