28. Implement strStr()

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public class Solution {
    public int strStr(String haystack, String needle) {
        if(haystack == null || needle == null || needle.length() > haystack.length()) {
            return -1;
        }else if(haystack.equals(needle) || needle.length() == 0) {
            return 0;
        }
        int index = -1;
        boolean[][] f = new boolean[needle.length()+1][haystack.length()+1];
        for(int j = 0; j < haystack.length()+1; j++) {
            f[0][j] = true;
        }
        for(int i = 1; i <= needle.length(); i++) {
            for(int j = 1; j <= haystack.length(); j++) {
                if(needle.charAt(i-1) == haystack.charAt(j-1) && f[i-1][j-1]) {
                    f[i][j] = true;
                    if(i == needle.length()) {
                        index = j-needle.length();
                        break;
                    }
                }
            }

        }
        return index;
    }

}