### 498. Diagonal Traverse

Given a matrix of M x N elements (M rows, N columns), return all elements of the matrix in diagonal order as shown in the below image.

Example:

Input:
[
[ 1, 2, 3 ],
[ 4, 5, 6 ],
[ 7, 8, 9 ]
]
Output:  [1,2,4,7,5,3,6,8,9]
Explanation:



Note:

1. The total number of elements of the given matrix will not exceed 10,000.

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