\def\fifo#1,#2,{\ifx\ofif#2$#1$\ofif\fi\proc{#1},{#2},\fifo#2,}

\def\ofif#1\ofif,{\fi}

\def\proc#1,#2,{$#1\ifnum#1>#2\searrow{}\else\nearrow{}\fi$}

\def\zigzag#1{\fifo#1,\ofif,}

\zigzag{1,-1,0,20,7,60,4,33,21,11,90,12345,67890}