将对称满阵a转化为稀疏存储：按行存储
寻找上三角阵首个非零元位置，上三角阵非零元所在列，非零元值，对角元值(Convert symmetric full array a into sparse storage: store by line
Find the first nonzero element position of the upper triangular array, the nonzero element of the upper triangular array, the nonzero element value, and the diagonal element value)