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 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.

题目意思：一个字符串s，它连续相同的字符数量所组成的新的字符串t，有s=t；

如S = “1221121221221121122....”，连续相同的字符数量有1个1，2个2，2个1，1个2，1个1.....，数量所组成的新的字符串t = "1221121..."，有s = t;

程序如下所示：

class Solution {
    public int magicalString(int n) {
        if (n == 0){
            return 0;
        }
        StringBuilder sb = new StringBuilder("122");
        int cnt = 1, index = 2;
        int cur = 0;
        while(true){
            cur = sb.charAt(index) - '0';
            if (cur == 1){
                cnt++;
            }
            char c = sb.charAt(sb.length() - 1) == '1'?'2':'1';
            for (int i = 0; i < cur; ++ i){
                sb.append(c);
            }
            index ++;
            if (index >= n){
                break;
            }
        }
        return cnt;
    }
}