十字链表结点的定义 根据i（行号）和j(列号)找出矩阵第i行第j列的非零元素在十字链表中的前驱结点-The definition of cross-linked list of nodes according to i (line number) and j (column number) to identify the precursor of the nonzero elements of the matrix i-th row j-th column in the cross-linked