LeetCode 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 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:

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Input: 6
Output: 3
Explanation: The first 6 elements of magical string S is "12211" and it contains three 1's, so return 3.

The original problem is here.

My Solution

I solve this problem in C++, as below:

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class Solution {
	public:
	    int magicalString(int n) {
	        if(n == 0)
	            return 0;
	        if(n <= 3){
	            return 1;
	        }
	        vector<int> str;
	        str.push_back(1);
	        str.push_back(2);
	        str.push_back(2);
	        int res=1;
	        int char12 = 1;
	        int count, j=2;
	        for(int i=3; i<n;i++) {
	            count = str[j];
	            for(int k=0; k<count; k++){
	                if (char12 == 1) {
	                    str.push_back(1);
	                    res++;
	                } else {
	                    str.push_back(2);
	                }
	                if(str.size() == n)
	                    return res;
	            }
	            char12 = 1 - char12;
	            j++;
	        }
	        return res;
	    }
};

Note

To solve the problem, save the history number and according to the history number to generate new number.