[leetcode] 481. Magical String

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 Sitself.

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.

这道题是统计特定字符串中‘1’的个数，题目难度为Medium。

一开始尝试找字符串的规律，看有没有重复或其他规律，不过没找到，不知道是否真的没有规律可循，找到规律的同学可以留言说明。

这里按规则生成字符串来统计‘1’的个数，具体规则很明确，没什么可讲的，注意最后统计‘1’的个数时不要直接统计整个字符串中‘1’的个数，因为生成的字符串长度可能已经超过N了。具体代码：

class Solution {public:    int magicalString(int n) {        if(n <= 0) return 0;        string s = "122";        int idx = 2;        while(s.size() < n)            s += string(s[idx++]-'0', s.back() ^ 3);        return count(s.begin(), s.begin()+n, '1');    }};

另外，字符串还可以以“22112...”开始，不过题目限定了以1开头，感兴趣的同学可以试试这种情况。