Today I have done leetcode's January LeetCoding Challenge with cpp.

January LeetCoding Challenge 3

Description

Number of Ways to Paint N × 3 Grid

You have a grid of size n x 3 and you want to paint each cell of the grid with exactly one of the three colors: Red, Yellow, or Green while making sure that no two adjacent cells have the same color (i.e., no two cells that share vertical or horizontal sides have the same color).

Given n the number of rows of the grid, return the number of ways you can paint this grid. As the answer may grow large, the answer must be computed modulo 109 + 7.

Example 1:

Input: n = 1
Output: 12
Explanation: There are 12 possible way to paint the grid as shown.

Example 2:

Input: n = 5000
Output: 30228214

Constraints:

• n == grid.length
• 1 <= n <= 5000

Solution

class Solution {
  const int MOD = 1000000007;
public:
  int numOfWays(int n) {
    long long A = 6, B = 6;

    for (int i = 2; i <= n; i++) {
      long long newA = (2 * A + 2 * B) % MOD;
      long long newB = (2 * A + 3 * B) % MOD;
      A = newA;
      B = newB;
    }

    return (A + B) % MOD;
  }
};

// Accepted
// 50/50 cases passed (2 ms)
// Your runtime beats 49.33 % of cpp submissions
// Your memory usage beats 70.13 % of cpp submissions (7.8 MB)