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

Find K Closest Elements Problem

LeetCode 658. Given a sorted integer array arr, two integers...

Count Binary Substrings Problem

LeetCode 696. Give a binary string s, return the number...

String Compression Problem

LeetCode 443. Given an array of characters chars, compress it...

Shortest Unsorted Continuous Subarray Problem

LeetCode 581. Given an integer array nums, you need to...

Reverse Words In A String III Problem

LeetCode 557. Given a string s, reverse the order of...

Reverse String II Problem

LeetCode 541. Given a string s and an integer k,...