459. Repeated Substring Pattern

Given a non-empty string check if it can be constructed by taking a substring of it and appending multiple copies of the substring together. You may assume the given string consists of lowercase English letters only and its length will not exceed 10000.

Example 1:

Input: "abab"

Output: True

Explanation: It's the substring "ab" twice.

Example 2:

Input: "aba"

Output: False

Example 3:

Input: "abcabcabcabc"

Output: True

Explanation: It's the substring "abc" four times. (And the substring "abcabc" twice.)

本题我开始想到了把字符串／2，然后让左面和右面进行比较，但是发现ababab这种的就没有办法解决了，看了答案，发现解决思路比较像，只不过把其最大公约数全部都比较一次，代码如下：

public class Solution {
    public boolean repeatedSubstringPattern(String str) {
        int len = str.length();
        for(int i=len/2;i>=1;i--){
            if(len%i==0){
                int p = len/i;
                String sub = str.substring(0,i);
                int k;
                for(k=0;k<p;k++){
                    if(!sub.equals(str.substring(k*i,k*i+i))) break;
                }
                if(k==p) return true;
            }
        }
        return false;
    }
}

 这道题的特点是，对于有很多重复的子字符串。