481. Magical String

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Problem

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 1:

Input: n = 6
Output: 3
Explanation: The first 6 elements of magical string s is "122112" and it contains three 1's, so return 3.

Example 2:

Input: n = 1
Output: 1

Constraints:

• 1 <= n <= 10^5

Code

class Solution {
    public int magicalString(int n) {
        if(n <= 3) return 1;
        
        int[] arr = new int[n];
        arr[0] = 1;
        arr[1] = 2;
        arr[2] = 2;
        
        int countIndex = 2;
        int endIndex = 3;
        int currNum = 1;
        int res = 1;
        
        while(endIndex < n){
            int currCount = arr[countIndex++];
            int i = 0;
            
            while(i < currCount && endIndex < n) {
                arr[endIndex] = currNum;
                endIndex++;
                i++;
                
                if(currNum == 1) res++;
            }
            
            currNum = currNum == 1 ? 2 : 1;
        }
        
        return res;
    }
}