Dsa Recursion All Subsets

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

Fix recursive subsets errors when result list shared reference causing all subsets to be same list.

Quick Fix

Wrong

def subsets(n):
    res=[]
    def dfs(i,cur):
        if i==len(n): res.append(cur); return
        dfs(i+1,cur)
        cur.append(n[i]); dfs(i+1,cur); cur.pop()
    dfs(0,[])
    return res

res.append(cur) appends reference, not copy. All entries become empty list after backtracking.

Right

def subsets(n):
    res=[]
    def dfs(i,cur):
        if i==len(n): res.append(cur.copy()); return
        dfs(i+1,cur)
        cur.append(n[i]); dfs(i+1,cur); cur.pop()
    dfs(0,[])
    return res

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

Prevention

Copy cur before adding to result. Recursion explores include/exclude for each element.

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