Buggy python submission (verdict: Runtime Error):
from collections import defaultdict


def can_palindrome(s):
    l = []
    for c in s:
        if c in l:
            l.remove(c)
        else:
            l.append(c)

    if len(s) % 2 == 0:
        return len(l) == 0
    else:
        return len(l) == 1


def can_pair(s1, s2):
    l = []
    for c1, c2 in zip(s1, s2):
        c = sorted([c1, c2])
        if c in l:
            l.remove(c)
        else:
            l.append(c)

    if len(s1) % 2 == 0:
        return len(l) == 0
    else:
        return len(l) == 1 and l[0][0] == l[0][1]


def pair_graph(s1, s2):
    N = len(s1)
    G = np.zeros([N, N])
    for i in range(N):
        for j in range(i+1, N):
            if s1[i] == s2[j] and s1[j] == s2[i]:
                G[i, j] = 1
                G[j, i] = 1
    return G

import numpy as np
H, W = list(map(int, input().split(' ')))
S = []
d = defaultdict(list)

for h in range(H):
    s = input()
    S.append(s)
    d[''.join(sorted(S))].append(h)

already_paired = np.zeros(H)

Gs = []

for i in range(H):
    if already_paired[i]:
        continue

    for j in range(i+1, H):
        if already_paired[j]:
            continue
    
        if can_pair(S[i], S[j]):
            already_paired[i] = 1
            already_paired[j] = 1
            G = pair_graph(S[i], S[j])
            Gs.append(G)
            break

if already_paired.sum() < H - 1:
    print('NO')
    exit()

elif already_paired.sum() == H - 1:
    for i in range(H):
        if already_paired[i] == 0:
            Gs.append(pair_graph(S[i], S[i]))
            break

def is_perfect_matching(G):
    N = len(G)
    if N == 1:
        return True
    for i in range(N):
        for j in range(i+1, N):
            if G[i, j] == 1:
                l = list(range(N))
                l.remove(i)
                l.remove(j)
                if len(l) == 0:
                    return True
                if is_perfect_matching(G[l][:, l]):
                    return True
    return False

if is_perfect_matching(G):
    print('YES')
else:
    print('NO')

Fixed submission:
def _3():
    from collections import defaultdict

    def can_pair(s1, s2):
        l = []
        for c1, c2 in zip(s1, s2):
            c = sorted([c1, c2])
            if c in l:
                l.remove(c)
            else:
                l.append(c)

        if len(s1) % 2 == 0:
            return len(l) == 0
        else:
            if len(l) == 1:
                if l[0][0] == l[0][1]:
                    return True
            return False

    def pair_graph(s1, s2):
        N = len(s1)
        G = np.zeros([N, N])
        for i in range(N):
            for j in range(i+1, N):
                if s1[i] == s2[j] and s1[j] == s2[i]:
                    G[i, j] = 1
                    G[j, i] = 1
        return G

    import numpy as np
    H, W = list(map(int, input().split(' ')))
    S = []
    d = defaultdict(list)

    for h in range(H):
        s = input()
        S.append(s)
        d[''.join(sorted(S))].append(h)

    already_paired = np.zeros(H)

    Gs = []

    for i in range(H):
        if already_paired[i]:
            continue

        for j in range(i+1, H):
            if already_paired[j]:
                continue

            if can_pair(S[i], S[j]):
                already_paired[i] = 1
                already_paired[j] = 1
                G = pair_graph(S[i], S[j])
                Gs.append(G)
                break

    if already_paired.sum() < H - 1:
        print('NO')
        exit()

    elif already_paired.sum() == H - 1:
        for i in range(H):
            if already_paired[i] == 0:
                G = pair_graph(S[i], S[i])
                Gs.append(G)
                break

    G = np.array(Gs).sum(axis=0) // len(Gs)

    def is_perfect_matching(G):
        N = len(G)
        if N == 1:
            return True

        for i in range(N):
            for j in range(i+1, N):
                if G[i, j] == 1:
                    l = list(range(N))
                    l.remove(i)
                    l.remove(j)
                    if len(l) == 0:
                        return True
                    if is_perfect_matching(G[l][:, l]):
                        return True
        return False

    if is_perfect_matching(G):
        print('YES')
    else:
        print('NO')

if __name__ == '__main__':
    _3()

Buggy python submission (verdict: Runtime Error):
import numpy as np
H, W = list(map(int, input().split(' ')))
S = []
d = defaultdict(list)

for h in range(H):
    s = input()
    S.append(s)
    d[''.join(sorted(S))].append(h)

already_paired = np.zeros(H)

Gs = []

for i in range(H):
    if already_paired[i]:
        continue

    for j in range(i+1, H):
        if already_paired[j]:
            continue

        if can_pair(S[i], S[j]):
            already_paired[i] = 1
            already_paired[j] = 1
            G = pair_graph(S[i], S[j])
            Gs.append(G)

if already_paired.sum() < H - 1:
    print('NO')
    exit()

elif already_paired.sum() == H - 1:
    for i in range(H):
        if already_paired[i] == 0:
            Gs.append(pair_graph(S[i], S[i]))
            break

G = np.array(Gs).sum(axis=0) // len(Gs)

def is_perfect_matching(G):
    N = len(G)
    for i in range(N):
        for j in range(i+1, N):
            if G[i, j] == 1:
                l = list(range(N))
                l.remove(i)
                l.remove(j)
                if len(l) == 0:
                    return True
                if is_perfect_matching(G[l][:, l]):
                    return True
    return False

if is_perfect_matching(G):
    print('YES')
else:
    print('NO')

Fixed submission:
def _3():
    from collections import defaultdict

    def can_pair(s1, s2):
        l = []
        for c1, c2 in zip(s1, s2):
            c = sorted([c1, c2])
            if c in l:
                l.remove(c)
            else:
                l.append(c)

        if len(s1) % 2 == 0:
            return len(l) == 0
        else:
            if len(l) == 1:
                if l[0][0] == l[0][1]:
                    return True
            return False

    def pair_graph(s1, s2):
        N = len(s1)
        G = np.zeros([N, N])
        for i in range(N):
            for j in range(i+1, N):
                if s1[i] == s2[j] and s1[j] == s2[i]:
                    G[i, j] = 1
                    G[j, i] = 1
        return G

    import numpy as np
    H, W = list(map(int, input().split(' ')))
    S = []
    d = defaultdict(list)

    for h in range(H):
        s = input()
        S.append(s)
        d[''.join(sorted(S))].append(h)

    already_paired = np.zeros(H)

    Gs = []

    for i in range(H):
        if already_paired[i]:
            continue

        for j in range(i+1, H):
            if already_paired[j]:
                continue

            if can_pair(S[i], S[j]):
                already_paired[i] = 1
                already_paired[j] = 1
                G = pair_graph(S[i], S[j])
                Gs.append(G)
                break

    if already_paired.sum() < H - 1:
        print('NO')
        exit()

    elif already_paired.sum() == H - 1:
        for i in range(H):
            if already_paired[i] == 0:
                G = pair_graph(S[i], S[i])
                Gs.append(G)
                break

    G = np.array(Gs).sum(axis=0) // len(Gs)

    def is_perfect_matching(G):
        N = len(G)
        if N == 1:
            return True

        for i in range(N):
            for j in range(i+1, N):
                if G[i, j] == 1:
                    l = list(range(N))
                    l.remove(i)
                    l.remove(j)
                    if len(l) == 0:
                        return True
                    if is_perfect_matching(G[l][:, l]):
                        return True
        return False

    if is_perfect_matching(G):
        print('YES')
    else:
        print('NO')

if __name__ == '__main__':
    _3()

Buggy python submission (verdict: Runtime Error):
H, W = list(map(int, input().split(' ')))
S = []
d = defaultdict(list)

for h in range(H):
    s = input()
    S.append(s)
    d[''.join(sorted(S))].append(h)

already_paired = np.zeros(H)

Gs = []

for i in range(H):
    if already_paired[i]:
        continue

    for j in range(i+1, H):
        if already_paired[j]:
            continue

        if can_pair(S[i], S[j]):
            already_paired[i] = 1
            already_paired[j] = 1
            G = pair_graph(S[i], S[j])
            Gs.append(G)

if already_paired.sum() < H - 1:
    print('NO')

elif already_paired.sum() == H - 1:
    for i in range(H):
        if already_paired[i] == 0:
            Gs.append(pair_graph(S[i], S[i]))
            break

G = np.array(Gs).sum(axis=0) // len(Gs)

def is_perfect_matching(G):
    N = len(G)
    for i in range(N):
        for j in range(i+1, N):
            if G[i, j] == 1:
                l = list(range(N))
                l.remove(i)
                l.remove(j)
                if len(l) == 0:
                    return True
                if is_perfect_matching(G[l][:, l]):
                    return True
    return False

print(is_perfect_matching(G))

Fixed submission:
def _3():
    from collections import defaultdict

    def can_pair(s1, s2):
        l = []
        for c1, c2 in zip(s1, s2):
            c = sorted([c1, c2])
            if c in l:
                l.remove(c)
            else:
                l.append(c)

        if len(s1) % 2 == 0:
            return len(l) == 0
        else:
            if len(l) == 1:
                if l[0][0] == l[0][1]:
                    return True
            return False

    def pair_graph(s1, s2):
        N = len(s1)
        G = np.zeros([N, N])
        for i in range(N):
            for j in range(i+1, N):
                if s1[i] == s2[j] and s1[j] == s2[i]:
                    G[i, j] = 1
                    G[j, i] = 1
        return G

    import numpy as np
    H, W = list(map(int, input().split(' ')))
    S = []
    d = defaultdict(list)

    for h in range(H):
        s = input()
        S.append(s)
        d[''.join(sorted(S))].append(h)

    already_paired = np.zeros(H)

    Gs = []

    for i in range(H):
        if already_paired[i]:
            continue

        for j in range(i+1, H):
            if already_paired[j]:
                continue

            if can_pair(S[i], S[j]):
                already_paired[i] = 1
                already_paired[j] = 1
                G = pair_graph(S[i], S[j])
                Gs.append(G)
                break

    if already_paired.sum() < H - 1:
        print('NO')
        exit()

    elif already_paired.sum() == H - 1:
        for i in range(H):
            if already_paired[i] == 0:
                G = pair_graph(S[i], S[i])
                Gs.append(G)
                break

    G = np.array(Gs).sum(axis=0) // len(Gs)

    def is_perfect_matching(G):
        N = len(G)
        if N == 1:
            return True

        for i in range(N):
            for j in range(i+1, N):
                if G[i, j] == 1:
                    l = list(range(N))
                    l.remove(i)
                    l.remove(j)
                    if len(l) == 0:
                        return True
                    if is_perfect_matching(G[l][:, l]):
                        return True
        return False

    if is_perfect_matching(G):
        print('YES')
    else:
        print('NO')

if __name__ == '__main__':
    _3()