Dsa Graph Cycledetect

In this tutorial, you'll learn about How to Fix Cycle Detection in Graph Errors. We cover key concepts, practical examples, and best practices.

Fix cycle detection in graph errors when DFS missing back-edge detection or visited tracking wrong.

Quick Fix

Wrong

def has_cycle(g):
    vis=set()
    def dfs(v):
        for n in g[v]:
            if n not in vis: vis.add(n); dfs(n)
            else: return True
        return False
    for v in g:
        if v not in vis: vis.add(v); dfs(v)
    return False

Doesn't distinguish back edge from cross edge. Returns false positives.

Right

def has_cycle(g):
    vis=set(); rec=set()
    def dfs(v):
        vis.add(v); rec.add(v)
        for n in g[v]:
            if n in rec: return True
            if n not in vis and dfs(n): return True
        rec.remove(v); return False
    for v in g:
        if v not in vis:
            if dfs(v): return True
    return False

Graph {0:[1,2],1:[2],2:[0,3]} -> True (cycle). O(V+E).

Prevention

Maintain recursion stack set. Back edge goes to node in current recursion stack.

DodaTech Tools

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