Given an m x n integer matrix, determine whether every top-left to bottom-right diagonal contains identical values. A diagonal of a matrix is a sequence of cells starting from some cell on the top row or left column and proceeding down-right (each next cell is one row down and one column right). The matrix is "diagonal-equal" (a Toeplitz matrix) if and only if all elements along each such diagonal are equal. Return true if every diagonal has all-equal values, otherwise false. Key observation: a cell (i, j) lies on the same diagonal as cell (i-1, j-1). So the whole matrix is diagonal-equal exactly when matrix[i][j] == matrix[i-1][j-1] for every cell that has an up-left neighbor (i >= 1 and j >= 1). This gives an O(m*n) time check with O(1) extra space. Edge cases to handle: an empty matrix (no rows, or rows with no columns) is trivially true; a single row or single column has no diagonal of length > 1 and is trivially true; values may be negative. Follow-up (space trade-off): if the matrix arrives one row at a time over a stream and you cannot revisit prior rows, keep only the previous row and compare each new row against it shifted by one, using O(n) extra space instead of O(1). Example 1: Input: matrix = [[1,2,3,4],[5,1,2,3],[9,5,1,2]] Output: true Explanation: The diagonals are [9], [5,5], [1,1,1], [2,2,2], [3,3], [4], each with identical values. Example 2: Input: matrix = [[1,2],[2,2]] Output: false Explanation: The main diagonal is [1,2], which is not all-equal.