Run the Floyd-Warshall algorithm on the weighted, directed graph of Figure 25.2. Show the matrix D(k) that results for each iteration of the outer loop.
straightforward.
Show how to compute the transitive closure using the technique of Section 25.1.
将EXTEND-SHORTEST-PATHS中第7行的min换成OR,+换成AND
Modify the FLOYD-WARSHALL procedure to include computation of the Π(k) matrices according to equations (25.6) and (25.7). Prove rigorously that for all i ∈ V , the predecessor subgraph Gπ,i is a shortest-paths tree with root i.
证明分三步：
-
无环。 每次循环过程中，一定有 dij(k) <= dij(k - 1)。 首先证明如果检验dij(k)之后有 πij(k) = l，则 dij(k) >= dil(k) + wlj。 检验dij(k)的过程中， 若 dij(k - 1) <= dik(k - 1) + dkj(k - 1)，有 dij(k) = dij(k - 1)，πij(k) = πij(k - 1) 都为l，l 存在于 (1, 2, ... , k - 1) 中。 k - 1 次循环在此时已经完成，故有 dij(k) = dij(k - 1) = dil(k - 1) + wlj >= dil(k) + wlj。
若 dij(k - 1) > dik(k - 1) + dkj(k - 1)，有 dij(k) = dik(k - 1) + dkj(k - 1)，必然有 πij(k) = πkj(k - 1) 都为l，l 存在于 (1, 2, ... , k - 1) 中。 k - 1 次循环在此时已经完成，故有 dij(k) = dik(k - 1) + dkj(k - 1) = dik(k - 1) + dkl(k - 1) + wlj，又因为dil(k - 1) <= dik(k - 1) + dkl(k - 1)（dil(k - 1) 是最短路径权重），有 dij(k) >= dil(k - 1) + wlj >= dil(k) + wlj。 现在假设Gπ,i中存在环路 (v0, v1, ... , vs)，vs = v0，πip(k) = p - 1，p = 1, 2, ... , s。不失一般性，假设 πis(k) = s - 1 是形成环的最后一步，这一步之前 πis(k) != s - 1。那么这一步之前一定有 dij(k) > dil(k) + wlj。此时，将所有 (v0, v1, ... , vs)对应的式子累加起来，有 Σdij(k) > Σdil(k) + Σwlj，j = 1, 2, ... , s，l = 0, 1, ... , s - 1，有Σwlj < 0，j = 1, 2, ... , s，l = 0, 1, ... , s - 1，这与Floyd-Wallshall算法基本假设 不存在权重为负的环 矛盾。
-
Gπ,i 是一棵以 i 为根的有根树。 因为 πij 记录了从 i 到 j 的路径中j的前驱点，Gπ,i 只取 πij 不为空的点，根据归纳法容易证明 Gπ,i 中存在 i 到 Gπ,i 中任意点的简单路径。下面证明这种简单路径唯一。 假设 i 到 j 有不止一条简单路径，设其中两条为 i~>p~>x->z~>j 和 i~>p~>y->z~>j，对于z点，有 πiz = x 同时 πiz = y，有 x = y，这两条路径为一条。同样的方法可以证明所有路径其实是一条，与假设矛盾。
-
Gπ,i 包含的一定是 i 到每个点的最短路径。根据算法过程可知正确。
As it appears above, the Floyd-Warshall algorithm requires Θ(n3) space, since we compute for i, j, k = 1, 2,...,n. Show that the following procedure, which simply drops all the superscripts, is correct, and thus only Θ(n2) space is required.
当然是正确的,因为这个动态规划只需要保存上一个状态.也就是要计算当前这个状态只需要借助上一个状态.
Suppose that we modify the way in which equality is handled in equation (25.7):
Is this alternative definition of the predecessor matrix Π correct?
感觉正确呀.
How can the output of the Floyd-Warshall algorithm be used to detect the presence of a negative-weight cycle?
只需要在正常的Floyd-Warshall算法完成后再多跑一个循环，如果有一个值还能更新则说明有负权回路。 或者，权重矩阵D对角线上出现了负值。
Another way to reconstruct shortest paths in the Floyd-Warshall algorithm uses values Φij(k) for i, j, k = 1, 2,..., n, where Φij(k) is the highest-numbered intermediate vertex of a shortest path from i to j in which all intermediate vertices are in the set {1, 2,..., k}. Give a recursive formulation for  Φij(k) , modify the FLOYD-WARSHALL procedure to compute the Φij(k) values, and rewrite the PRINT-ALL-PAIRS-SHORTEST-PATH procedure to take the matrix Φ = (Φij(n)) as an input. How is the matrix Θ like the s table in the matrix-chain multiplication problem of Section 15.2?
Φij(k－1) 如果dij(k-1) <= dik(k-1) + dkj(k-1)
Φij(k) ＝
k otherwise
PRINT-ALL-PAIRS-SHORTEST-PATH(Φ,i,j)
if i == j
then print i
else if Φ(i,j) = -1
then print "no path from 'i' to 'j' exists"
else
PRINT-ALL-PAIRS-SHORTEST-PATH(Φ,i,Φ(i, j))
PRINT-ALL-PAIRS-SHORTEST-PATH(Φ,Φ(i, j),j)Give an O(V E)-time algorithm for computing the transitive closure of a directed graph G = (V, E).
对每个节点都跑一次DFS. 一共V个点,E条边，所以是O(VE)的时间.
Suppose that the transitive closure of a directed acyclic graph can be computed in f(|V|,|E|) time, where f is a monotonically increasing function of |V| and |E|. Show that the time to compute the transitive closure G* = (V, E*) of a general directed graph G = (V, E) is f(|V|,|E|) + O(V + E*).
首先随便选择一个点开始DFS，搜索中如果遇到灰色的点说明有环，把这一次搜索用到的边 (u, v) 记录下来，并从E中删除。这个操作复杂度 O(V + E) <= O(V + E*)。这个操作之后有环图变成了无环图，运行f(|V|, |E|)算法，得到不完整的传递闭包。
结束后，遍历被删除的边，连接u和v，以及所有v能到达的点。由于记录 (u, v) 不会重复，遍历过程最多把不完整的传递闭包每条边遍历一遍，外加被删除的边（这些边一定存在于E*中），因此复杂度为O(E*)。
因此，这个算法总的复杂度为f(|V|, |E|) + O(V + E*)。
Follow @louis1992 on github to help finish this task.