📜 WAYPOINT LESSON
Constrained Lattice Paths
⭐⭐⭐ advanced⏳ 30 min read📍 Lesson 175 of 281
Total paths minus paths through the obstacle: a two-binomial product that the reflection identity makes O(1).
Subtracting the bad routes
Paths on an n×m grid from corner to corner: C(n+m, n). With ONE forbidden cell: total - paths((0,0)→cell) × paths(cell→goal). Each factor is a single binomial. Multiple obstacles need inclusion-exclusion over subsets — the previous lesson generalizes.
⚡ Now practice
Ready to CodeCounting SuiteCatalans at n = 5e5, four-divisor inclusion-exclusion at N = 1e12, and lattice paths around a forbidden cell.
3 challenges · · ~110 min