java, 261 bytes

Result

AC

Duration

1334ms

Code [DL]

interface A{static void main(String[]a)  throws java.io.IOException {
int c=100,d=0;
	for(;c>10;){
		c=System.in.read()&0xff;
		if(c>=65){
			d=c; System.out.write(c);
		}
		else{
			for(;c>49;c--){
				System.out.write(d);
			}
		}
	}
	System.out.write(10);
}}

stdin

k8n4j9g6h4j9b3u3k8c9y2h5i3t3r9a7x7l4d6v6f9e3s4e9u9s7w7l8kg6o7t4rf3l2x9n2m5g4e6q2r9j2p7g2x5gwez2

stdout

kkkkkkkknnnnjjjjjjjjjgggggghhhhjjjjjjjjjbbbuuukkkkkkkkcccccccccyyhhhhhiiitttrrrrrrrrraaaaaaaxxxxxxxllllddddddvvvvvvfffffffffeeesssseeeeeeeeeuuuuuuuuussssssswwwwwwwllllllllkggggggooooooottttrfffllxxxxxxxxxnnmmmmmggggeeeeeeqqrrrrrrrrrjjpppppppggxxxxxgwezz

stderr

strace