Leetcode 481 - Magical String

Note:

• Use recursion to generate strings then count.
• Use s to contain the result, and group[] to contain every group.
• At first, there will be missing elements in group[]. Add them alternatively based on last element of groups[], which means if the last is 1 we add 2.
• But how many times should we repeat?
• The patterns is s[groups.length], which is the first num that groups[] is missing.

Question:

A magical string s consists of only '1' and '2' and obeys the following rules:

• The string s is magical because concatenating the number of contiguous occurrences of characters '1' and '2' generates the string s itself.

The first few elements of s is s = "1221121221221121122……". If we group the consecutive 1’s and 2’s in s, it will be "1 22 11 2 1 22 1 22 11 2 11 22 ......" and the occurrences of 1’s or 2’s in each group are "1 2 2 1 1 2 1 2 2 1 2 2 ......". You can see that the occurrence sequence is s itself.

Given an integer n, return the number of 1‘s in the first n number in the magical string s.

Example:

Code: