Implement strStr() @leetcode

StrStr.java

package leetcode.simpleCoding;

/**
 * @date Feb 8, 2014
 * 
 * Test case:  (1123 123)  ("","") --> may OutOfBound ,("a","a") --> s1.length()- s2.length()+1 not without +1, bound check. 
 * return value: haystack.subString(i) but not String.valueOf(i) ("","" will return 0).
 * take care 
 */
public class StrStr {

	public static void main(String args[]) {
		String haystack = "a";
		String needle = "a";
		System.out.println(strStr(haystack, needle));
	}
	
	public static String strStr(String haystack, String needle) {
		if (haystack == null || needle == null) {
			return null;
		}

		int i,j = 0;
		
		for (i = 0; i < haystack.length() - needle.length() + 1; i++) {
			
			for (j = 0; j<needle.length(); j++) {
				if (haystack.charAt(i + j) != needle.charAt(j)) {
					break;
				}
			}
			
			if (j == needle.length()) {
				return haystack.substring(i);
			}
		}

		return null;
	}
}