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 string S is the following: 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 above is the S itself.
Given an integer N as input, return the number of '1's in the first N number in the magical string S.
Note: N will not exceed 100,000.
Example 1:
Input: 6 Output: 3 Explanation: The first 6 elements of magical string S is "12211" and it contains three 1's, so return 3.
1 public class Solution { 2 public int magicalString(int n) { 3 if (n <= 0) return 0; 4 if (n < 4) return 1; 5 6 int[] magical = new int[n]; 7 magical[0] = 1; magical[1] = magical[2] = 2; 8 9 int result = 1; 10 int start = 2, end = 2; 11 while (start < n) { 12 if (magical[start] == 1) { 13 if (end + 1 < n) 14 magical[end + 1] = magical[end] == 1 ? 2 : 1; 15 16 end++; 17 result += 1; 18 } else { 19 if (end + 2 < n) 20 magical[end + 2] = magical[end] == 1 ? 2 : 1; 21 if (end + 1 < n) 22 magical[end + 1] = magical[end] == 1 ? 2 : 1; 23 end += 2; 24 } 25 start++; 26 } 27 return result; 28 } 29 }