差分约束建边方法

1.求最大值，用最短路，不等式转化成 a <= b + c形式，从b向a建立权值为c的边g[b].append((a,c))

2.求最小值，用最长路，不等式转化成a >= b + c形式，从b向a建立权值为c的边

import sys
from collections import defaultdict, deque

from math import inf, sqrt

RI = lambda: int(sys.stdin.readline().strip())
RS = lambda: sys.stdin.readline().strip()
RII = lambda: map(int, sys.stdin.readline().strip().split())
RILIST = lambda: list(RII())

g = defaultdict(list)
n,ml,md = RII()
# 求最大值，用最短路，a <= b + c形式表示
# 按顺序排列，要求 i <= (i + 1) + 0
for i in range(1,n + 1):
    g[i + 1].append((i,0))
# b - a <= l  b <= a + l
for _ in range(ml):
    a,b,l = RII()
    if b < a:
        a,b = b,a
    g[a].append((b,l))
# b - a >= d  a <= b - d
for _ in range(md):
    a,b,d = RII()
    if b < a:
        a,b = b,a
    g[b].append((a,-d))

dis = [inf]*(n + 1)

def spfa(k):
    # 最短路，找负环
    global dis
    dis = [inf]*(n + 1)
    cnt = [0]*(n + 1)
    st = [0]*(n + 1)
    q = deque()
    for i in range(1,k + 1):
        dis[i] = 0
        st[i] = 1
        q.append(i)
    while q:
        u = q.popleft()
        st[u] = 0
        for v,w in g[u]:
            if dis[v] > dis[u] + w:
                dis[v] = dis[u] + w
                cnt[v] = cnt[u] + 1
                if cnt[v] >= n:
                    return True
                if not st[v]:
                    st[v] = 1
                    q.append(v)
    return False

if spfa(n):
    print(-1)
else:
    spfa(1)
    if dis[n] == inf:
        print(-2)
    else:
        print(dis[n])