[leetcode]481. Magical String

题目链接：https://leetcode.com/PRoblems/lexicographical-numbers/

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: 6Output: 3Explanation: The first 6 elements of magical string S is "12211" and it contains three 1's, so return 3.
class Solution {public:    int magicalString(int n) {        string s = "122";        int index = 2;        while(s.length() < n) {            int cnt = s[index] - '0';            char c = (s.back() == '1' ? '2' : '1');            string temp(cnt, c);            s += temp;            index++;        }        return count(s.begin(), s.begin() + n, '1');    }};