# # 【LeetCode】481. Magical String 解题报告（Python）

## # 题目描述：

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.

## # 解题方法

S的开头一定是1,2,2；合并就是1个1、2个2，所以到了第三个数字，这个数字是2，所以一定是2个1，故S的下两位是1，即S是1,2,2,1,1；到了S的第四个数字是1，所以下面是1个2；即S是1,2,2,1,1,2……

class Solution(object):
def magicalString(self, n):
"""
:type n: int
:rtype: int
"""
S = [1, 2, 2]
idx = 2
while len(S) < n:
S += [3 - S[-1]] * S[idx]
idx += 1
return S[:n].count(1)

https://leetcode.com/problems/magical-string/discuss/96472/Short-Python-using-queue

## # 日期

2018 年 9 月 7 日 ———— 中午不睡，下午崩溃