847A - Union of Doubly Linked Lists - CodeForces Solution

def getEnd(i):
    while nodes[i][1] != 0: i = nodes[i][1]-1
    return i

n = int(input())
nodes = [list(map(int, input().split())) for _ in range(n)]
starts = [i for i in range(n) if nodes[i][0] == 0]
ends = [getEnd(i) for i in starts]

for i in range(len(starts)-1):
    nodes[ends[i]][1] = starts[i+1]+1
    nodes[starts[i+1]][0] = ends[i]+1
    
for row in nodes: print(*row)