Give a N * N square matrix, return all the elements of its anti-diagonals from top to bottom.

Example 1:

Input: 
N = 2
A = [[1, 2],
     [3, 4]]
Output:
1 2 3 4
Explanation: Topmost anti-diagonal is [[1, ], 
                                       [ , ]]
Next anti-diagonal is [[ , 2], 
​                       [3,  ]]
and the last anti-diagonal is [[ ,  ], 
​                               [ , 4]]
Hence, elements will be returned in the 
order {1, 2, 3, 4}.

Example 2:

Input: 
N = 3 
A = [[1, 2, 3],
     [4, 5, 6],
     [7, 8, 9]]
Output: 
1 2 4 3 5 7 6 8 9

Your Task:
You don't need to read input or print anything. Your task is to complete the function downwardDigonal() which takes an integer N and a 2D matrix A[ ][ ] as input parameters and returns the list of all elements of its anti-diagonals from top to bottom.

Expected Time Complexity: O(N*N)
Expected Auxillary Space: O(N*N)

Constraints:
1 ≤ N, M ≤ 10³
0 ≤ A[i][j] ≤ 10⁶