Fragile network

import sys
from collections import deque, defaultdict, Counter
from heapq import heappush, heappop, heapify
from bisect import bisect_left, bisect_right
from itertools import permutations, combinations, product, accumulate, groupby
from functools import lru_cache, cmp_to_key, reduce
from math import gcd, isqrt, comb, inf, pi, sqrt, atan2, hypot


def main():
    data = sys.stdin.buffer.read().split(); idx = 0
    n = int(data[idx]); idx += 1
    # n, m = map(int, data[idx:idx + 2]); idx += 2
    # a = list(map(int, data[idx:idx + n])); idx += n
    # s = data[idx].decode(); idx += 1                  # strings


    conn_count = [0] * (n+1)

    for _ in range(n-1):
        a, b = map(int, data[idx:idx + 2]); idx += 2

        conn_count[a] += 1
        conn_count[b] += 1


    



    # g = [data[idx + i].decode() for i in range(n)]; idx += n  # grid of n rows, no spaces, strings

    # input = sys.stdin.readline
    # n, m = map(int, input().split())
    # s = input().strip()  # strip removes the trailing newline
    # for line in sys.stdin:  # read until EOF

    leafs = []
    out = []

    for i in range(1, n+1):
        if conn_count[i] == 1:
            leafs.append(str(i))

    # print(len(leafs))

    for i in range(0, len(leafs) -1, 2):
        # print(*leafs[i:i+2])
        out.append(" ".join(leafs[i:i+2]))

    if len(leafs) %2 != 0:
        out.append(" ".join(leafs[len(leafs)-2:len(leafs)]))
    print(len(out))



    sys.stdout.write("\n".join(map(str, out)) + "\n")  # one value per line
    # print(*out)                                      # space separated on one line
    # print(f"{ans:.6f}")                              # float with 6 decimals
    # print("\n".join("".join(row) for row in g))      # grid of chars


main()

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: WRONG ANSWER

Test 11

Verdict: ACCEPTED

Test 12

Verdict: WRONG ANSWER

Test 13

Verdict: WRONG ANSWER

Test 14

Verdict: WRONG ANSWER

Test 15

Verdict: WRONG ANSWER

Test 16

Verdict: WRONG ANSWER

Test 17

Verdict: WRONG ANSWER

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: ACCEPTED

Test 21

Verdict: ACCEPTED