KMP算法

next：最长相等前后缀~~ 次长

class Solution {
    public int strStr(String haystack, String needle) {
        int n = haystack.length();
        int m = needle.length();
        if(m == 0){
            return 0;
        }
        int[] next = new int[m];
        for(int i = 1, j = 0; i < m; i++){
            while(j > 0 && needle.charAt(i) != needle.charAt(j)){
                j = next[j - 1];
            }
            if(needle.charAt(i) == needle.charAt(j)){
                j++;
            }
            next[i] = j;
        }
        for(int i = 0, j = 0; i < n; i++){
            while(j > 0 && haystack.charAt(i) != needle.charAt(j)){
                j = next[j - 1];
            }
            if(haystack.charAt(i) == needle.charAt(j)){
                j++;
            }
            if(j == m){
                return i - m + 1;
            }
        }
        return -1;

    }
}