[leetcode-779-K-th Symbol in Grammar]

Posted 2020-10-24 hellowOOOrld

On the first row, we write a 0. Now in every subsequent row, we look at the previous row and replace each occurrence of 0 with 01, and each occurrence of 1 with 10.

Given row N and index K, return the K-th indexed symbol in row N. (The values of K are 1-indexed.) (1 indexed).

Examples:
Input: N = 1, K = 1
Output: 0

Input: N = 2, K = 1
Output: 0

Input: N = 2, K = 2
Output: 1

Input: N = 4, K = 5
Output: 1

Explanation:
row 1: 0
row 2: 01
row 3: 0110
row 4: 01101001

Note:

1. N will be an integer in the range [1, 30].
2. K will be an integer in the range [1, 2^(N-1)].

思路：

递归即可，判断当前的k是奇数还是偶数，如果是偶数，那么再查看上一行是1还是0即可。k是奇数同理。

int kthGrammar(int N, int K) {
      if(N ==  1) return  0;
      if(K % 2 == 0) return (kthGrammar(N - 1, K / 2) == 0) ? 1 : 0;
      else   return (kthGrammar(N - 1, (K+1) / 2) == 0) ? 0 : 1; 
    }