bash-busybox, 71 bytes

Result

WA

Duration

1151ms

Code [DL]

fold -b1|awk '{if(+$1){for(;--$1;)print a}else{print a=$1}}'|tr -d '\n'

stdin

zkg5t4a2eco3z2x4w5t6zgfb7b9c8er8g8l4r5j4p8d8l3s9ym8q2x3u6z3dn5y9h9s6a7t4r5s5p4i4t6i8v3p9u4

stdout

stderr

/code.bash: line 1: /code.bash: line 1: tr: command not found
/code.bash: line 1: awk: command not found
fold: command not found

strace