779. K-th Symbol in Grammar

We build a table of n rows (1-indexed). We start by writing 0 in the 1st row. 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.

• For example, for n = 3, the 1st row is 0, the 2nd row is 01, and the 3rd row is 0110.

Given two integer n and k, return the kth (1-indexed) symbol in the nth row of a table of n rows.

class Solution {
public:
    int kthGrammar(int n, int k) {
        int len = 1<<(n-1);
        bool reverse = false;
        while(len != 1) {
            if(k > len / 2) {
                k = len - k + 1;
                if(!(n & 1))
                    reverse = !reverse;
            }
            len>>=1;
            n--;
        }
        return reverse ? 1 : 0;
    }
};