1065.c

#include <stdio.h>

#define BLANK -1
#define SIGNED -2
#define SINGLE -3

int main()
{
	int couple[100000], count = 0, N, M, ID1, ID2;
	for (int i = 0; i < 100000; i++)
		couple[i] = BLANK;

	/* Read 'couple-list', every pair of 'index' and 'value' are a couple. */
	scanf("%d", &N);
	for (int i = 0; i < N; i++) {
		scanf("%d %d", &ID1, &ID2);
		couple[ID1] = ID2;
		couple[ID2] = ID1;
	}

	scanf("%d", &M);
	for (int i = 0; i < M; i++) {        /* Read guest list. */
		scanf("%d", &ID1);
		if (couple[ID1] >= 0)                /* If one has a mate */
			couple[ID1] = SIGNED;           /*   set SIGNED */
		else                                /* Else: not in the 'couple-list' */
			couple[ID1] = SINGLE, count++;  /*   set SINGLE */
	}

	/* If couple[ID] is >= 0 (but not signed) but couple[couple[ID]] == SIGNED
	 * (signed in), this means 'ID' didn't come but his/her mate did.
	 * So his/her mate is alone in the party, set to SINGLE. */
	for (int i = 0; i < 100000; i++)
		if (couple[i] >= 0 && couple[couple[i]] == SIGNED)
			couple[couple[i]] = SINGLE, count++;

	/* Those whose value is SINGLE is either a bachelor or came alone */
	printf("%d\n", count);
	for (int i = 0; i < 100000; i++)
		if (couple[i] == SINGLE)
			printf("%05d%c", i, --count ? ' ' : '\0');

	return 0;
}