Exercise 3: Count Sink Nodes 🔚

Your Task

Write a function count_sinks(num_nodes, edges) that counts how many nodes have no outgoing edges.

A node with no outgoing edges is called a sink — nothing flows out of it.

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Examples

print(count_sinks(4, [[0,1], [0,2], [1,3], [2,3]]))
# Output: 1
# Only node 3 has no outgoing edges

print(count_sinks(3, [[0,1], [1,2], [2,0]]))
# Output: 0
# Every node has an outgoing edge

print(count_sinks(3, []))
# Output: 3
# No edges at all — every node is a sink

print(count_sinks(5, [[0,1], [0,2]]))
# Output: 4
# Nodes 1, 2, 3, 4 have no outgoing edges

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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.

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Approach

1. Build the adjacency list
2. Count nodes where the neighbor list is empty

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Starter Code

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

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

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

print(count_sinks(3, []))
# Output: 3

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

print(count_sinks(6, [[0,1], [1,2], [2,3], [3,4], [4,5]]))
# Output: 1

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Hints