leetcode:Implement strStr()

Implement strStr().

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

题目的要求就是实现KMP算法

public class Solution {
    public String strStr(String haystack, String needle) {
        //if(haystack.equals("") && needle.equals("")) return "";
        if(needle.equals("")) return haystack;
        String ans = null;
        int i = 0, j = 0;
        int next[] = getNext(needle);
        while(i < haystack.length() && j < needle.length()){
            if(j == -1 || haystack.charAt(i) == needle.charAt(j)){
                ++j; ++i;
                
            }else{
                j = next[j];
            }
        }
        if(j != 0 && j == needle.length()){
            ans = new String(haystack.toCharArray(), i - j, haystack.length() - i + j );
        }
        return ans;
    }
    
    private int[] getNext(String str){
        if(str.length() > 0){
            int[] next = new int[str.length()];
            next[0] = -1;
            int len = str.length() - 1;
            int i = 0, k = -1;
            while(i < len){
                if(k == -1 || str.charAt(i) == str.charAt(k)){
                    ++i; ++k;
                    next[i] = k;
                }else{
                    k = next[k];
                }
            }
            return next;
        }
        return null;
    }
}