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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:**

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

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