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CoursesBacktracking & Greedy

Backtracking Foundations

Lessons
9
Exercises
57
Minutes
61
  1. 1

    Choose, Explore, Unchoose

    Backtracking walks a decision tree: commit to a branch, explore it fully, then undo — the current path lives on a stack.

    Code orderMultiple choice
    6 exercises
    5 min
  2. 2

    Letter Case Permutation

    Two branches per letter, one per digit — walk the case tree over a shared char array and record a string at every leaf.

    Complete codeDebugMultiple choiceTrace
    6 exercises
    6 min
  3. 3

    Sum of All Subset XOR Totals

    Include or exclude each element — 2^n leaves, one subset per leaf, XOR along the path and sum across the leaves.

    Complete codeMultiple choiceTrace
    7 exercises
    7 min
  4. 4

    Letter Combinations of a Phone Number

    Multi-way branching: each digit opens one level of the tree, and each of its letters is one branch — depth equals the number of digits.

    Complete codeMultiple choiceTrace
    6 exercises
    6 min
  5. 5

    Permutations

    Order matters: every unused element branches at every level, tracked by used[] flags — and every mark must be undone on the way back.

    Complete codeDebugMultiple choiceTrace
    6 exercises
    7 min
  6. 6

    Generate Parentheses

    Constrained branching: add '(' while open < n and ')' while close < open — invalid prefixes are never built, so no filtering is needed at the leaves.

    Complete codeDebugMultiple choiceTrace
    6 exercises
    7 min
  7. 7

    Backtracking CheckpointCheckpoint

    No new concepts — transfer the island's branching moves to unseen problems: choose-k LED selections, adjacency-constrained strings, and digit chains where pruning collapses the whole tree.

    Code orderMultiple choiceTrace
    6 exercises
    7 min
  8. 8

    Backtracking ReviewReview

    Mixed recap of the island: the choose/explore/unchoose contract, include/exclude vs used[] branching, why pruning beats filtering, copy-on-record, stack depth vs tree size, and 2^n vs n!.

    Complete codeMultiple choice
    6 exercises
    6 min
  9. 9

    Boss: SubsetsBoss

    The island's final test: enumerate the power set. Every element is one independent in/out decision — 2^n subsets including [] — and the boss's trap is recording a reference to the mutable path instead of a copy.

    Complete codeDebugCode orderMultiple choiceTrace
    8 exercises
    10 min
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Backtracking & Greedy is part of AlgoFox Pro

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