Fragile network

#include <stdint.h>
#include <stdlib.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>

typedef struct cell{
    uint64_t val;
    struct cell *next;
    struct cell *prev;
} cell;

uint64_t *status;
uint64_t *values;
cell **calls;
uint64_t n, m, k;
cell *lc, *end;

void add(uint64_t id){
    cell *c = (cell*) malloc(sizeof(cell));
    c->val=id;
    if(lc != NULL){
        lc->prev = c;
    } else {
        end = c;
    }
    c->next = lc;
    c->prev = NULL;
    lc = c;
}

void explore(uint64_t id){
    // printf("test : %ld\n", id);
    if (status[id] == 1){
        printf("No\n");
        exit(EXIT_SUCCESS);
    }
    if (status[id] == 2){
        return;
    }
    if (calls[id] == NULL){
        k++;
        status[id] = 2;
        add(id);
        return;
    }
    status[id] = 1;
    cell *cur = calls[id];
    while (cur != NULL){
        explore(cur->val);
        cur = cur->next;
    }
    status[id] = 2;
    add(id);
}

int main() {
    if (scanf("%ld", &n) == 0)
        return EXIT_FAILURE;
    m=n-1;
    k=0;

    calls = (cell**) calloc(n, sizeof(cell*));

    uint64_t a, b;
    cell *c;
    for (uint64_t i = 0; i<m; i++){
        if (scanf("%ld %ld", &a, &b) == 0)
            return EXIT_FAILURE;
        a--;
        b--;
        c = (cell*) malloc(sizeof(cell));
        c->prev = NULL;
        c->next = calls[a];
        c->val = b;
        calls[a] = c;
    }

    status = (uint64_t*) calloc(n, sizeof(uint64_t));
    values = (uint64_t*) calloc(n, sizeof(uint64_t));

    for (uint64_t i = 0; i<n; i++){
        explore(i);
    }
    printf("%ld\n", (k+1)/2);
    cell *prev_left = lc;
    cell *left = lc;
    cell *right = end;
    while (right != left){
        while (calls[left->val] != NULL && left != right)
            left = left->next;
        if (right == left){
            printf("%ld %ld\n", prev_left->val+1, right->val+1);
            exit(EXIT_SUCCESS);
        }
        while (calls[right->val] != NULL && left != right)
            right = right->prev;
        printf("%ld %ld\n", left->val+1, right->val+1);
        prev_left = left;
        left = left->next;
        if (right == left){
            printf("%ld %ld\n", prev_left->val+1, right->val+1);
            exit(EXIT_SUCCESS);
        }
        right = right->prev;
    }




    return EXIT_SUCCESS;
}

Test 1

Verdict: WRONG ANSWER

Test 2

Verdict: ACCEPTED

Test 3

Verdict: WRONG ANSWER

Test 4

Verdict: ACCEPTED

Test 5

Verdict: WRONG ANSWER

Test 6

Verdict: WRONG ANSWER

Test 7

Verdict: ACCEPTED

Test 8

Verdict: RUNTIME ERROR

Test 9

Verdict: WRONG ANSWER

Test 10

Verdict: WRONG ANSWER

Test 11

Verdict: WRONG ANSWER

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: WRONG ANSWER

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED

Test 18

Verdict: WRONG ANSWER

Test 19

Verdict: ACCEPTED

Test 20

Verdict: WRONG ANSWER

Test 21

Verdict: WRONG ANSWER