正确答案应该是多少

把 0 到 3 的所有路径都枚举一遍，取最短的。运行下面这段程序：

def brute(g, s, t):
    INF = 10 ** 9
    best = [INF]
    def go(u, c, seen):
        if u == t:
            if c < best[0]:
                best[0] = c
            return
        for v, w in g[u]:
            if v not in seen:
                go(v, c + w, seen | {v})
    go(s, 0, {s})
    return best[0]

print(brute({0: [(1, 4), (2, 5)], 1: [(3, 1)], 2: [(1, -3)], 3: []}, 0, 3))