501B - Misha and Changing Handles - CodeForces Solution

positions_old = {}
positions_new = {}
olds = []
news = []

q = int(input())
for vez in range(q):
    old, new = input().split()
    if old in positions_new:
        position = positions_new[old]
        news[position] = new
        del positions_new[old]
        positions_new[new] = position
    else:
        position = len(olds)
        olds.append(old)
        news.append(new)
        positions_old[old] = position
        positions_new[new] = position
    
num_users = len(olds)
print(num_users)
for position in range(num_users):
    print(f'{olds[position]} {news[position]}')