/ zigzag conversion

g:{y,&|/z=\:x}
f:{y()g[(!#y)!m]/+(x#!m;x#|!1+m:-2+x*2)}    / h/t charles hall

"PINALSIGYAHRPI"~f[4]"PAYPALISHIRING"
"PAHNAPLSIIGYIR"~f[3]"PAYPALISHIRING"