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; } };
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