Magical String Problem


Description

LeetCode Problem 481.

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:

1
2
3
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:

1
2
Input: n = 1
Output: 1

Constraints:

  • 1 <= n <= 10^5


Sample C++ Code

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
class Solution {
public:
    int magicalString(int n) {
        if (n < 1)
            return 0;
        
        queue<int> q;
        int ans = 1, num = 1, count = 2, i = 3;
        while (i < n) {
            while (count--) {
                q.push(num);
                if (i < n && num == 1)
                    ++ans;
                ++i;
            }
            
            num = (num == 1 ? 2 : 1);
            count = q.front();
            q.pop();
        }
        return ans;
    }
};




Related Posts

Squares Of A Sorted Array Problem

LeetCode 977. Given an integer array nums sorted in non-decreasing...

Sort Array By Parity Problem

LeetCode 905. Given an integer array nums, move all the...

Sort Array By Parity II Problem

LeetCode 922. Given an array of integers nums, half of...

Shortest Distance To A Character Problem

LeetCode 821. Given a string s and a character c...

Reverse Only Letters Problem

LeetCode 917. Given a string s, reverse the string according...

Push Dominoes Problem

LeetCode 838. There are n dominoes in a line, and...