ImplementStrstr.java

//If use startWith, it's easy
public class Solution {
    public String strStr(String haystack, String needle) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(needle.length() == 0)
            return haystack;
        for(int i = 0; i < haystack.length(); ++i){
            if(haystack.substring(i).startsWith(needle))
                return haystack.substring(i);
        }
        return null;
    }
}

//without using startsWith()
public class Solution {
    public String strStr(String haystack, String needle) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(needle.length() == 0)
	    return haystack;
        for(int i = 0; i < haystack.length() - needle.length() + 1; ++i){
            for(int j = 0; j < needle.length(); ++j){
                if(haystack.charAt(i + j) != needle.charAt(j))  
		    break;
                if(j == needle.length() - 1)
		    return haystack.substring(i);
            }
        }
        return null;
    }
}