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Problems

Basics first, then the classics, then company-style assessments. Every problem has tests you run right here; multi-level ones unlock as you go. See the roadmap.

Trees

Tree DP

Post-order: each node returns a small tuple of states to its parent.

Visual guide Quiz (9) Spot the pattern
Notes

Recognise it when: you're optimizing over a tree with choices per node (take or skip, place a camera or not) or paths through nodes.

def dfs(node):                     # returns (best if robbed, best if not)
    if not node: return (0, 0)
    l, r = dfs(node.left), dfs(node.right)
    rob = node.val + l[1] + r[1]
    skip = max(l) + max(r)
    return (rob, skip)

Maximum path sum: return the best downward path (never negative, so clamp at 0), and keep a global best of node + left + right.

Gotchas: separate "what I return to my parent" from "what I record as the answer". Watch the recursion depth.

10 problems

Interview roadmap

Trees Traversals, BSTs, and answers built bottom-up.

Tree DP guide

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