在保留+或 - 符号的同时找到最大绝对值

If I have a matrix:

如果我有一个矩阵:

mat=matrix(c(-21,14,28,17,-16,-9,-17,-30,18), nrow=3)
mat
          [,1]   [,2]  [,3]
     [1,]  -21     17    17
     [2,]   14    -16   -30
     [3,]   28     -9    18
mat=matrix(c(