Buggy python submission (verdict: Time Limit Exceeded):
n, m = map(int, input().split())
lrd = []

MAX = 999999

p = [[] for _ in range(n)]
for i in range(m):
    l, r, d = map(int, input().split())
    lrd.append([l-1, r-1, d, 0])
    p[l-1].append(i)
    p[r-1].append(i)

if m <= 1:
    print('Yes')
    exit()

a = [MAX for _ in range(n)]

a[lrd[0][0]] = 0
temp = [lrd[0][0]]
next = []
t = 0
s = 0
max_a = 0
min_a = 0
while 1:
    next = []
    for x in temp:
        for y in p[x]:
            if lrd[y][0] == x:
                t = lrd[y][1]
                s = a[x] + lrd[y][2]
            else:
                t = lrd[y][0]
                s = a[x] - lrd[y][2]

            lrd[y][3] = 1
            if a[t] == MAX:
                a[t] = s
                next.append(t)
            elif a[t] != s:
                print('No')
                exit()

    if len(next) == 0:
        target = 0
        i = 0
        check = 0
        for i in range(m):
            if lrd[i][3] == 0:
                target = lrd[i][0]
                check = 1
        if check == 0:
            break
        a[target] = 0
        next = [target]

    temp = [] + list(set(next))

print('Yes')


Fixed submission:
n, m = map(int, input().split())
lrd = []

MAX = 999999

p = [[] for _ in range(n)]
for i in range(m):
    l, r, d = map(int, input().split())
    lrd.append([l-1, r-1, d, 0])
    p[l-1].append(i)
    p[r-1].append(i)

if m <= 1:
    print('Yes')
    exit()

a = [MAX for _ in range(n)]


target = lrd[0][0]
a[target] = 0
temp = [target]
t = 0
while 1:
    next = []
    for x in temp:
        for y in p[x]:
            if lrd[y][3] == 1:
                continue

            if lrd[y][0] == x:
                t = lrd[y][1]
                d = a[x] + lrd[y][2]
            else:
                t = lrd[y][0]
                d = a[x] - lrd[y][2]

            lrd[y][3] = 1

            if a[t] == MAX:
                a[t] = d
                next.append(t)
            elif a[t] != d:
                print('No')
                exit()

    if len(next) == 0:
        check = 0
        for i in range(m):
            if lrd[i][3] == 0:
                check = 1
                target = lrd[i][0]
                break

        if check == 1:
            temp = [target]
            a[target] = 0
            continue
        else:
            break

    temp = [] + list(set(next))

print('Yes')

Buggy python submission (verdict: Time Limit Exceeded):
n, m = map(int, input().split())
lrd = []

MAX = 999999

p = [[] for _ in range(n)]
for i in range(m):
    l, r, d = map(int, input().split())
    lrd.append([l-1, r-1, d, 0])
    p[l-1].append(i)
    p[r-1].append(i)

if m <= 1:
    print('Yes')
    exit()

a = [MAX for _ in range(n)]

a[lrd[0][0]] = 0
temp = [lrd[0][0]]
next = []
t = 0
s = 0
max_a = 0
min_a = 0
while 1:
    next = []
    #print("temp", temp)
    for x in temp:
        for y in p[x]:
            if lrd[y][0] == x:
                t = lrd[y][1]
                s = a[x] + lrd[y][2]
            else:
                t = lrd[y][0]
                s = a[x] - lrd[y][2]

            lrd[y][3] = 1
            if a[t] == MAX:
                a[t] = s
                next.append(t)
            elif a[t] != s:
                #print(temp, t, a[t])
                print('No')
                exit()

    #print(next)
    if len(next) == 0:
        target = 0
        i = 0
        check = 0
        for i in range(m):
            if lrd[i][3] == 0:
                target = lrd[i][0]
                check = 1
        if check == 0:
            break
        a[target] = 0
        next = [target]

    #print(next)
    temp = [] + list(set(next))

print('Yes')


Fixed submission:
n, m = map(int, input().split())
lrd = []

MAX = 999999

p = [[] for _ in range(n)]
for i in range(m):
    l, r, d = map(int, input().split())
    lrd.append([l-1, r-1, d, 0])
    p[l-1].append(i)
    p[r-1].append(i)

if m <= 1:
    print('Yes')
    exit()

a = [MAX for _ in range(n)]


target = lrd[0][0]
a[target] = 0
temp = [target]
t = 0
while 1:
    next = []
    for x in temp:
        for y in p[x]:
            if lrd[y][3] == 1:
                continue

            if lrd[y][0] == x:
                t = lrd[y][1]
                d = a[x] + lrd[y][2]
            else:
                t = lrd[y][0]
                d = a[x] - lrd[y][2]

            lrd[y][3] = 1

            if a[t] == MAX:
                a[t] = d
                next.append(t)
            elif a[t] != d:
                print('No')
                exit()

    if len(next) == 0:
        check = 0
        for i in range(m):
            if lrd[i][3] == 0:
                check = 1
                target = lrd[i][0]
                break

        if check == 1:
            temp = [target]
            a[target] = 0
            continue
        else:
            break

    temp = [] + list(set(next))

print('Yes')