481. Magical String ----M

 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:

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

A:

就是构造出来就好了嘛

（先加入3个，是避免处理下一个没有数量的情况）

class Solution {
public:
    int magicalString(int n) {
        vector<int> S;
        S.push_back(1);
        S.push_back(2);
        S.push_back(2);
        int countIndex = 1;
        while(S.size()<n){
            // count how many and added it.
            int nextValue = S.back()==1?2:1;
            int nextCount = S[++countIndex];
            for(int i =0;i<nextCount;i++){
                S.push_back(nextValue);
            }
        }
        int res = 0;
        for(int i =0;i<n;i++)   {
            if(S[i]==1){
                res++;
            }
        }
        return res;
    }
};