Merkkijono

#include <stdio.h>

char S[1024];
int main(void) {
	for (int t=1,c; (S[t] = ~(c=getchar())?c:0);)
		if (S[--t] == c || (t+=2, 0))
			while ((c=getchar())==S[t+1] || ungetc(c,stdin)-c);
	puts(S+1);
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED