Buggy python submission (verdict: Runtime Error):
# import numpy as np
# import queue
# import heapq


def solve(writer):
    N, A, B, C = map(int, input().split())
    variables = {
        'A': A,
        'B': B,
        'C': C
    }

    queries = (list(input()) for _ in range(N))

    buffer = list()
    b = ''

    for i in range(N):
        if not buffer:
            t1, t2 = next(queries)
        else:
            b = buffer.pop()
            if b == 'pass':
                continue
            else:
                t1, t2 = list(b)

        v1 = variables[t1]
        v2 = variables[t2]

        if v1 == v2 == 0:
            return False
        elif v1 == v2 == 1:
            if i == N-1:
                return True

            nt1, nt2 = next(queries)
            if len({[t1, t2, nt1, nt2]}) == 2:
                buffer.append('pass')
                writer.append(t1)
                writer.append(t2)
                continue

            if t1 in {nt1, nt2}:
                variables[t1] += 1
                variables[t2] -= 1
                writer.append(t1)
            else:
                variables[t1] -= 1
                variables[t2] += 1
                writer.append(t2)

            buffer.append(''.join([nt1, nt2]))

        elif v1 > v2:
            variables[t1] -= 1
            variables[t2] += 1
            writer.append(t2)
        elif v2 > v1:
            variables[t1] += 1
            variables[t2] -= 1
            writer.append(t1)
        else:
            variables[t1] += 1
            variables[t2] -= 1
            writer.append(t1)

    return True


def main():
    writer = list()
    ok = solve(writer)
    if ok:
        print('Yes')
        for w in writer:
            print(w)
    else:
        print('No')


if __name__ == "__main__":
    main()


Fixed submission:
def solve(writer):
    N, A, B, C = map(int, input().split())
    variables = {
        'A': A,
        'B': B,
        'C': C
    }

    queries = (list(input()) for _ in range(N))

    buffer = list()
    b = ''

    for i in range(N):
        if not buffer:
            t1, t2 = next(queries)
        else:
            b = buffer.pop()
            if b == 'pass':
                continue
            else:
                t1, t2 = list(b)

        v1 = variables[t1]
        v2 = variables[t2]

        if v1 == v2 == 0:
            return False
        elif v1 == v2 == 1:
            if i == N-1:
                writer.append(t1)
                return True

            nt1, nt2 = next(queries)
            if len({t1, t2, nt1, nt2}) == 2:
                buffer.append('pass')
                writer.append(t1)
                writer.append(t2)
                continue

            if t1 in {nt1, nt2}:
                variables[t1] += 1
                variables[t2] -= 1
                writer.append(t1)
            else:
                variables[t1] -= 1
                variables[t2] += 1
                writer.append(t2)

            buffer.append(''.join([nt1, nt2]))

        elif v1 > v2:
            variables[t1] -= 1
            variables[t2] += 1
            writer.append(t2)
        elif v2 > v1:
            variables[t1] += 1
            variables[t2] -= 1
            writer.append(t1)
        else:
            variables[t1] += 1
            variables[t2] -= 1
            writer.append(t1)

    return True


def main():
    writer = list()
    ok = solve(writer)
    if ok:
        print('Yes')
        for w in writer:
            print(w)
    else:
        print('No')


if __name__ == "__main__":
    main()