LeetCode in Kotlin

481. Magical String

Medium

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

Solution

class Solution {
    fun magicalString(n: Int): Int {
        val a = IntArray(n + 2)
        var fast = 1
        var slow = 1
        var num = 1
        while (fast <= n) {
            a[fast++] = num
            if (a[slow++] == 2) {
                a[fast++] = num
            }
            num = 3 - num
        }
        var count = 0
        for (j in 1..n) {
            if (a[j] == 1) {
                count++
            }
        }
        return count
    }
}

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