How to Fix Permutations Errors

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

Fix permutations errors when shared list object mutates after adding to result or duplicates not handled.

Quick Fix

Wrong

def perm(n):
    res=[]
    def dfs(cur):
        if len(cur)==len(n): res.append(cur); return
        for x in n:
            if x not in cur: dfs(cur+[x])
    dfs([])
    return res

cur+[x] creates new list each call. Fine for small n. O(n*n!) time.

Right

def perm(n):
    res=[]; used=[False]*len(n)
    def dfs(cur):
        if len(cur)==len(n): res.append(cur.copy()); return
        for i,x in enumerate(n):
            if not used[i]:
                used[i]=True; cur.append(x); dfs(cur); cur.pop(); used[i]=False
    dfs([])
    return res

[1,2,3] -> [[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]]. O(n*n!).

Prevention

Track used indices. For each position, try each unused element. Backtrack after.

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