KMP真的难，先做个记录吧

28. 找出字符串中第一个匹配项的下标

class Solution {
    private int[][] dp;
    private String pat;
    
    // 构建 DP 表
    private void buildDp(String pat) {
        int M = pat.length();
        dp = new int[M][256];
        dp[0][pat.charAt(0)] = 1;
        int X = 0;
        for (int j = 1; j < M; j++) {
            for (int c = 0; c < 256; c++) {
                dp[j][c] = dp[X][c];
            }
            dp[j][pat.charAt(j)] = j + 1;
            X = dp[X][pat.charAt(j)];
        }
    }

    // KMP 搜索函数
    public int search(String txt) {
        int M = pat.length();
        int N = txt.length();
        int j = 0;
        for (int i = 0; i < N; i++) {
            j = dp[j][txt.charAt(i)];
            if (j == M) return i - M + 1;
        }
        return -1;
    }

    // 用 KMP 实现的 strStr 函数
    public int strStr(String haystack, String needle) {
        if (needle == null || needle.length() == 0) {
            return 0;
        }
        if (haystack == null || haystack.length() == 0) {
            return -1;
        }
        this.pat = needle;
        buildDp(needle);
        return search(haystack);
    }
}

459. 重复的子字符串

判断字符串s是否由重复子串组成，只要两个s拼接在一起，里面还出现一个s的话，就说明是由重复子串组成。

class Solution {
    public boolean repeatedSubstringPattern(String s) {
        String tmp = s + s;
        tmp = tmp.substring(1, tmp.length() - 1);
        if(tmp.indexOf(s) > -1){
            return true;
        }
        return false;
    }
}