Implement strStr(). Java

LeetCode

Implement strStr().

Returns a pointer to the first occurrence of needle in haystack, or null if needle is not part of haystack.

	/*
	Implement strStr().
	Returns a pointer to the first occurrence of needle in haystack,
	or null if needle is not part of haystack.
	*/
	public class StrStr {
	public String strStr(String haystack, String needle) {
	if (haystack==null||needle==null){
	return null;
	}
	if (haystack.length()==0){
	if (needle.length()==0){
	return haystack;
	}
	else{
	return null;
	}
	}
	if (needle.length()==0){
	return haystack;
	}
	if (haystack.length()<needle.length()){
	return null;
	}
	// below code is a modified version of code in /**/ below, idea from a google+ friend.
	if (i>=0){
	return haystack.substring(i);
	}
	else{
	return null;
	}
	/*
	int i=0;
	int len=haystack.length()-needle.length();
	while(i<=len){
	if (haystack.charAt(i)==needle.charAt(0)){
	if (haystack.substring(i, i+needle.length()).equals(needle)){
	return haystack.substring(i);
	}
	}
	i++;
	}
	return null;
	*/
	}
	}
	// without use substring()
	public class Solution {
	public String strStr(String haystack, String needle) {
	// Start typing your Java solution below
	// DO NOT write main() function
	assert(haystack!=null && needle!=null);
	if (needle.length()==0){
	return haystack;
	}
	int i=0;
	while (i<haystack.length()){
	if (haystack.length()-i<needle.length()){
	break;
	}
	if (haystack.charAt(i)==needle.charAt(0)){
	int j=i;
	while (j-i<needle.length()&&needle.charAt(j-i)==haystack.charAt(j)){
	j++;
	if (j-i==needle.length()){
	return haystack.substring(i);
	}
	}
	}
	i++;
	}
	return null;
	}
	}

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