给定一个有向无环图 (DAG),有N个顶点和M 个边,任务是打印从入度为零的顶点开始的所有路径。
Indegree of a vertex is the total number of incoming edges to a vertex.
例子:
Input: N = 6, edges[] = {{5, 0}, {5, 2}, {4, 0}, {4, 1}, {2, 3}, {3, 1}}
Output: All possible paths:
4 0
4 1
5 0
5 2 3 1
Explaination:
The given graph can be represented as:
There are two vertices whose indegree are zero i.e vertex 5 and 4, after exploring these vertices we got the fallowing path:
4 -> 0
4 -> 1
5 -> 0
5 -> 2 -> 3 -> 1
Input: N = 6, edges[] = {{0, 5}}
Output: All possible paths:
0 5
Explaination:
There will be only one possible path in the graph.
方法:
- 上述问题可以通过探索所有入度0的顶点和印刷在路径中的所有顶点,直到遇到具有出度0顶点来解决。按照以下步骤打印所有可能的路径:
- 创建一个布尔数组indeg0 ,它存储所有入度为零的顶点的真值。
- 对所有入度为 0 的顶点应用 DFS。
- 打印从入度为 0 的顶点到出度为零的顶点的所有路径。
Illustration:
For the above graph:
indeg0[] = {False, False, False, False, True, True}
Since indeg[4] = True, so appling DFS on vertex 4 and printing all path terminating to 0 outdegree vertex are as follow:
4 -> 0
4 -> 1
Also indeg[5] = True, so appling DFS on vertex 5 and printing all path terminating to 0 outdegree vertex are as follow:
5 -> 0
5 -> 2 -> 3 -> 1
下面是上述方法的实现:
Python3
# Python program to print all
# possible paths in a DAG
# Recursive function to print all paths
def dfs(s):
# Append the node in path
# and set visited
path.append(s)
visited[s] = True
# Path started with a node
# having in-degree 0 and
# current node has out-degree 0,
# print current path
if outdeg0[s] and indeg0[path[0]]:
print(*path)
# Recursive call to print all paths
for node in adj[s]:
if not visited[node]:
dfs(node)
# Remove node from path
# and set unvisited
path.pop()
visited[s] = False
def print_all_paths(n):
for i in range(n):
# for each node with in-degree 0
# print all possible paths
if indeg0[i] and adj[i]:
path = []
visited = [False] * (n + 1)
dfs(i)
# Driver code
from collections import defaultdict
n = 6
# set all nodes unvisited
visited = [False] * (n + 1)
path = []
# edges = (a, b): a -> b
edges = [(5, 0), (5, 2), (2, 3),
(4, 0), (4, 1), (3, 1)]
# adjacency list for nodes
adj = defaultdict(list)
# indeg0 and outdeg0 arrays
indeg0 = [True]*n
outdeg0 = [True]*n
for edge in edges:
u, v = edge[0], edge[1]
# u -> v
adj[u].append(v)
# set indeg0[v] <- false
indeg0[v] = False
# set outdeg0[u] <- false
outdeg0[u] = False
print('All possible paths:')
print_all_paths(n)
All possible paths:
4 0
4 1
5 0
5 2 3 1
时间复杂度: O (N + E) 2
辅助空间: O(N)
如果您想与行业专家一起参加直播课程,请参阅Geeks Classes Live