108_strStr.java

class Solution {
    /**
     * Returns a index to the first occurrence of target in source,
     * or -1  if target is not part of source.
     * @param source string to be scanned.
     * @param target string containing the sequence of characters to match.
     */
    public int strStr(String source, String target) {
        //write your code here
      if(source==null||target==null)
            return -1;
        int len1 = source.length();
        int len2 = target.length();
        if(len2==0)
            return 0;
        for(int i=0; i<=len1-len2; i++){
            for(int j=0; j<len2; j++){
                if(source.charAt(i+j)!=target.charAt(j))
                    break;
                if(j==len2-1)
                    return i;
            }
        }
        return -1;
    }
}