Path Sum III

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Solution Guide: Path Sum III

Approach

For each node, count paths starting there plus recurse into all descendants as new starts.

Complexity Analysis

Time
O(n^2) worst-case (simple DFS variant)
Space
O(h) recursion depth

Pattern

Root-Any-to-Any Path Sums

Why It Works

Every node can be a path start, and each call checks all downward continuations.

Updated Mar 2026