strstr

public class Solution {
public String strStr(String haystack, String needle) {
 
    int nLen = needle.length();
    int hLen = haystack.length();
    
    if(nLen==hLen && nLen==0)
        return "";
    if(nLen == 0)
        return haystack;
    for(int i=0; i<= hLen-nLen; i++){
        int j = 0;
        for( ; j<nLen;j++){
            if(needle.charAt(j)!=haystack.charAt(i+j))break;
        }
        if(j==nLen)
            return haystack.substring(i);
    }
    
    return null;
 
   
}

}