0481. Magical String

https://leetcode.com/problems/magical-string

Description

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 <= 105

ac

class Solution {
    public int magicalString(int n) {
        // edge cases
        if (n == 0) return 0;
        if (n < 3) return 1;

        int[] nums = new int[n];
        nums[0] = 1; nums[1] = nums[2] = 2;
        int i = 3, cnt1 = 1, countIdx = 2, curr = 1;

        while (i < n) {
            int count = nums[countIdx++];
            while (i < n && count-- > 0) {
                nums[i] = curr;
                if (nums[i] == 1) cnt1++;
                i++;
            }
            curr = 3 - curr; // flip 1 and 2
        }

        return cnt1;
    }
}

/*
1) build the array, when nums[i] is 1 cnt++; 2) flip curr between 1 and 2 each time, get count from array;
*/