from collections import deque def scc(N, G, RG): order = [] used = [0]*N def dfs(s): used[s] = 1 for t in G[s]: if not used[t]: dfs(t) order.append(s) for i in range(N): if not used[i]: dfs(i) group = [-1]*N label = 0 order.reverse() for s in order: if group[s] != -1: continue que = deque([s]) group[s] = label while que: v = que.popleft() for w in RG[v]: if group[w] != -1: continue que.append(w) group[w] = label label += 1 return group # topological ordering N = int(input()) G = [[] for i in range(2*N)] RG = [[] for i in range(2*N)] tri = [tuple(map(int, input().split())) for _ in range(N)] from collections import defaultdict dict = defaultdict(lambda:[]) def add_edge(i, neg_i, j, neg_j): if neg_i: i0 = i+N; i1 = i else: i0 = i; i1 = i+N if neg_j: j0 = j+N; j1 = j else: j0 = j; j1 = j+N # add (~a ⇒ b) G[i1].append(j0); RG[j0].append(i1) # add (~b ⇒ a) G[j1].append(i0); RG[i0].append(j1) for i in range(N): a, b, c = tri[i] for u, v in [(a, b), (b, c)]: dict[(u, v)].append((i, 1)) dict[(a, c)].append((i, 0)) for u, v in dict: if len(dict[(u, v)]) >= 3: print("NO") exit() elif len(dict[(u, v)]) == 2: i0, t0 = dict[(u, v)][0] i1, t1 = dict[(u, v)][1] if t0 == t1: add_edge(i0, True, i1, False) add_edge(i0, False, i1, True) else: add_edge(i0, True, i1, True) add_edge(i0, False, i1, False) group = scc(2*N, G, RG) # check if the formula is satisfiable def check(group): for i in range(N): if group[i] == group[i+N]: return False return True if check(group): print("YES") else: print("NO")