Dsa Graph Topological Sort

In this tutorial, you'll learn about How to Fix Topological Sort Errors. We cover key concepts, practical examples, and best practices.

Fix topological sort errors when cycle detection missing or ordering direction reversed.

Quick Fix

Wrong

def topo(g):
    def dfs(v):
        for n in g[v]: dfs(n)
        res.append(v)
    res=[]
    for v in g: dfs(v)
    return res[::-1]

No cycle detection. Infinite recursion on cyclic graph.

Right

def topo(g):
    def dfs(v):
        vis.add(v); rec.add(v)
        for n in g[v]:
            if n in rec: raise ValueError('cycle')
            if n not in vis: dfs(n)
        rec.remove(v); res.append(v)
    vis=set(); rec=set(); res=[]
    for v in g:
        if v not in vis: dfs(v)
    return res[::-1]

Graph {5:[2,0],4:[0,1],2:[3],3:[1],1:[],0:[]} -> [5,4,2,3,1,0]. O(V+E).

Prevention

Track recursion stack to detect cycles. Append after processing all dependencies.

DodaTech Tools

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