function K=Fast_Matrix_Assembly(Elements)

Input:
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Elements: a structure contains Elements{i}.K and Elements{i}.DOFs (Stiffness and Degrees of freedoms of the ith element).

output:
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K: Sparse global stiffness matrix

References
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Cuvelier, François, Caroline Japhet, and Gilles Scarella. "An efficient way to perform the assembly of finite element matrices in Matlab and Octave." arXiv preprint arXiv:1305.3122 (2013).