779. K-th Symbol in Grammar

Medium (Trung bình) C++ 🔗 Xem trên LeetCode

📋 Đề Bài

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.

 

Example 1:

Input: n = 1, k = 1
Output: 0
Explanation: row 1: 0

Example 2:

Input: n = 2, k = 1
Output: 0
Explanation: 
row 1: 0
row 2: 01

Example 3:

Input: n = 2, k = 2
Output: 1
Explanation: 
row 1: 0
row 2: 01

 

Constraints:

  • 1 <= n <= 30
  • 1 <= k <= 2n - 1

🧠 Thuật Toán & Kỹ Thuật

Bit Manipulation (Thao tác bit)
⏱️ Thời gian O(n)
💾 Không gian O(1)

💻 Lời Giải

C++ 0779-k-th-symbol-in-grammar.cpp 
class Solution {
public:
    int kthGrammar(int n, int k) {
        return __builtin_popcount(k - 1) & 1;
    }
};