[459]repeated substring pattern

repeated substring pattern

This problem can be solved using KMP table by O(n).

kmp by july

idea

if the string has repeated pattern, then it must has a common prefix and suffix. that is the same thing when we implement KMP.

Then we will first calculated the longest common-fix table.

Then we can judge by:

common != 0 && len % (len - common) == 0;

code

public class Solution{

    public boolean repeatedSubstringPattern(String str) {
        int len = str.length();
        if(len == 0)return true;
        int[] table = presuf(str);
        int common = table[len - 1];
        return common != 0 && len % (len - common) == 0;
    }
    public int[] presuf(String s){

        int len = s.length();

        int[] table = new int[len];
        if(len == 0)return table;


        for(int i = 1; i < len; i++){
            int last = table[i - 1];
            while(true){

                if(s.charAt(i) == s.charAt(last)){
                    table[i] = last + 1;
                    break;
                } else {
                    if(last == 0)break;
                    last = table[last - 1];

                }
            }

        }
        return table;

    }

}