算法-KMP(看毛片) JAVA版本

紧用来记录自己理解并且能手撸的代码，已过leetcode题目所有测试案例

public class KMP {

    public int[] next(char[] str2){
        if(str2.length == 1){
            return new int[]{-1};
        }
        int[] next = new int[str2.length];
        next[0] = -1;
        next[1] = 0;
        int i = 2;
        int cn = 0;
        while (i < str2.length){
            if(str2[i - 1] == str2[cn]){
                next[i++] = ++cn;
            }else {
                if(cn > 0){
                    cn = next[cn];
                }else {
                    next[i++] = 0;
                }
            }
        }
        return next;
    }

    public int strStr(String haystack, String needle){
        if((haystack != null && haystack.equals(needle)) ||
                (needle != null && needle.length() < 1) ){
            return 0;
        }
        if(haystack == null || needle == null || haystack.length() < needle.length()){
            return -1;
        }
        char[] chars1 = haystack.toCharArray();
        char[] chars2 = needle.toCharArray();
        int[] next = next(chars2);
        int i1 = 0, i2 = 0;
        while (i1 < chars1.length && i2 < chars2.length){
            if(chars1[i1] == chars2[i2]){
                i1++;
                i2++;
            }else if(next[i2] == -1 ){
                i1++;
            }else {
                i2 = next[i2];
            }
        }
        return i2 == chars2.length ? i1 - chars2.length : -1;

    }
}