kmp字符串匹配

         next数组求解

    public int[] makeNext(char[] arr) {
        int[] next = new int[arr.length];
        next[0] = -1;
        int k = -1, j = 0;
        while (j < arr.length - 1){
             // k 前缀  j后缀
            if (k == -1 || arr[j] == arr[k]) {
                if (arr[j+1] == arr[k+1]){
                    next[++j] = next[++k];
                } else{
                    next[++j] = ++k; 

                }
            } else {
                k = next[k];
            }
        }
        return next;
    }

        求 next数组的过程可以认为是将模式串拆分成n个子串，分别对每个子串求前缀和后缀的最长公共匹配字符数 l

	public int match(String haystack, String needle) {
		char[] arr = needle.toCharArray();
		int[] next = makeNext(arr);
		for (int i = 0, j = 0; i < haystack.length();){
			if (j == -1 || haystack.charAt(i) == arr[j]){
				i++;j++;
				if (j == arr.length) return i - arr.length;
			} else {
				 j = next[j];
			}
		}
		return -1;
	}