Leetcode008 -- implement strstr() function

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class test08 {
    /**
     *  Realization strStr() function .
     *  Here are two strings haystack  and  needle , Please come in  haystack  Find in string  needle  The first place the string appears （ Subscript from  0  Start ）. If it doesn't exist , Then return to  -1 .
     *
     *  explain ：
     *  When needle When it's an empty string , What value should we return ？ This is a good question in an interview .
     *  For this question , When needle When it's an empty string, we should return  0 . This is related to  C  Linguistic strstr() as well as  Java  Of indexOf() The definition matches .
     *
     *  Example  1：
     *  Input ：haystack = "hello", needle = "ll"
     *  Output ：2
     *
     *  Example  2：
     *  Input ：haystack = "aaaaa", needle = "bba"
     *  Output ：-1
     *
     *  Example  3：
     *  Input ：haystack = "", needle = ""
     *  Output ：0
     *
     *  Tips ：
     * 0 <= haystack.length, needle.length <= 5 * 104
     * haystack  and  needle  It only consists of lowercase English characters 
     * @param args
     */
    public static void main(String[] args) throws IOException {
        // Create a buffered character input stream 
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String haystack = br.readLine();
        String needle = br.readLine();
        System.out.println(strStr(haystack,needle));
    }
/**
 *  Positive thinking 
 */
//    public static int strStr(String haystack, String needle){
//        if(needle.length() == 0){
//            return 0;
//        }
//        int flag = -1;
//        for(int i=0; i<haystack.length(); i++){
//            if(haystack.charAt(i) == needle.charAt(0)){
//                if(needle.length() == 1){
//                    return i;
//                }else if( (i+needle.length()-1) >= haystack.length()){
//                    return flag;
//                }else{
//                    for(int j=1; j<needle.length(); j++){
//                        if(haystack.charAt(i+j) == needle.charAt(j)){
//                            if(j==needle.length()-1){
//                                flag = i;
//                                return flag;
//                            }else{
//                                flag = -1;
//                                continue;
//                            }
//                        }else {
//                            flag = -1;
//                            break;
//                        }
//                    }
//                }
//            }
//        }
//        return flag;
//    }

    /**
     *  Reverse thinking 
     * @param haystack
     * @param needle
     * @return
     */
    public static int strStr(String haystack, String needle) {
        int n = haystack.length(), m = needle.length();
        for (int i = 0; i + m <= n; i++) {
            boolean flag = true;
            for (int j = 0; j < m; j++) {
                if (haystack.charAt(i + j) != needle.charAt(j)) {
                    flag = false;
                    break;
                }
            }
            if (flag) {
                return i;
            }
        }
        return -1;
    }
}