算法-KMP-leetcode-java

原理：http://www.ruanyifeng.com/blog/2013/05/Knuth%E2%80%93Morris%E2%80%93Pratt_algorithm.html

示例：leetcode:Implement strStr()

 

public class Solution {
    public String strStr(String haystack, String needle) {

        int i = 0;
        int hlength = haystack.length();
        int nlength = needle.length();

        if (nlength == 0) {
            return haystack;
        } else {
            int[] form = KMPform(needle);

            while (i <= hlength - nlength) {
                int j = 0;
                while (j < nlength) {
                    if (haystack.charAt(i + j) != needle.charAt(j)) {
                        if (j == 0) {
                            i++;
                        } else {
                            i = i + j - form[j - 1]-1;
                        }
                        break;
                    } else if (j == nlength - 1) {
                        return haystack.substring(i);
                    }
                    j++;
                }
            }
            return null;
        }
    }
    
    public int[] KMPform(String str) {
        int[] form = new int[str.length()];
        form[0] = -1;
        for (int i = 1; i < form.length; i++) {
            int index = form[i - 1];
            while (index >= 0 && str.charAt(i) != str.charAt(index + 1))
            {
                index = form[index];
            }
            if (str.charAt(i) == str.charAt(index + 1))
            {
                form[i] = index + 1;
            }
            else
            {
                form[i] = -1;
            }
        }
        return form;
    }

}