Exercise 2: Get Neighbors of a Node 🔍

Your Task

Write a function get_neighbors(num_nodes, edges, x) that:

Builds an adjacency list from the given edge list
Returns the list of direct neighbors of node x (nodes that x points to)

The graph is directed — edge [u, v] means there's an arrow from u to v.

Examples

print(get_neighbors(4, [[0,1], [0,2], [1,3], [2,3]], 0))
# Output: [1, 2]

print(get_neighbors(4, [[0,1], [0,2], [1,3], [2,3]], 1))
# Output: [3]

print(get_neighbors(4, [[0,1], [0,2], [1,3], [2,3]], 3))
# Output: []

print(get_neighbors(3, [[0,1], [1,2], [2,0]], 2))
# Output: [0]

Expected Complexity

	Complexity
Time	O(V + E)
Space	O(V + E)

Where V is the number of nodes and E is the number of edges.

Why? Building the adjacency list takes O(V + E). Looking up neighbors is O(1).

Approach

Build the adjacency list (same as Exercise 1)
Return adj[x]

Starter Code

def get_neighbors(num_nodes, edges, x):
    # Your code here
    pass

# Test your function:
print(get_neighbors(4, [[0,1], [0,2], [1,3], [2,3]], 0))
# Output: [1, 2]

print(get_neighbors(4, [[0,1], [0,2], [1,3], [2,3]], 1))
# Output: [3]

print(get_neighbors(4, [[0,1], [0,2], [1,3], [2,3]], 3))
# Output: []

print(get_neighbors(3, [[0,1], [1,2], [2,0]], 2))
# Output: [0]

print(get_neighbors(5, [[0,1], [0,4], [1,2], [1,3]], 0))
# Output: [1, 4]

Hints

💡 Hint 1 — Build the adjacency list first

Reuse the same pattern from Exercise 1:

adj = {i: [] for i in range(num_nodes)}
for a, b in edges:
    adj[a].append(b)

💡 Hint 2 — Returning neighbors

Once you have adj, the neighbors of node x are just:

return adj[x]

Since we initialized every node with an empty list, even nodes with no outgoing edges are safe to look up.

✅ Solution

def get_neighbors(num_nodes, edges, x):
    adj = {i: [] for i in range(num_nodes)}
    for a, b in edges:
        adj[a].append(b)
    return adj[x]

Why does this work? The adjacency list maps each node to its outgoing neighbors. We initialize all nodes to [] first so that nodes with no outgoing edges (like node 3 in the first example) still return an empty list instead of raising a KeyError.

Get Neighbors of a Node