Implement strStr()

Implement strStr().

Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

 

public class Solution {
    public int strStr(String haystack, String needle) {
    	if (haystack.equals(needle) || needle.equals("")) {
    		return 0;
    	}
    	if (haystack == null || needle== null) {
    		return -1;
    	}
    	
    	int si = 0;
    	int mi = 0;
    	int[] next = getNextArray(needle);
    	while (si < haystack.length() && mi < needle.length()) {
    		if (haystack.charAt(si) == needle.charAt(mi)) {
    			si++;
    			mi++;
    		} else if (next[mi] == -1) {
    			si++;
    		} else {
    			mi = next[mi];
    		}
    	}
    	return mi == needle.length() ? si - mi : -1;
    }

	private int[] getNextArray(String needle) {
		// TODO Auto-generated method stub
		if (needle.length() == 1) {
			return new int[]{-1};
		}
		int[] next = new int[needle.length()];
		next[0] = -1;
		next[1] = 0;
		int pos = 2;
		int cn = 0;
		while (pos < next.length) {
			if (needle.charAt(pos-1) == needle.charAt(cn)) {
				next[pos++] = ++cn;
			} else if (cn > 0) {
				cn = next[cn];
			} else {
				next[pos++] = 0;
			}
		}
		return next;
	}
}