DP — Minimum Path Sum

solve(grid) — grid holds non-negative numbers. Starting at the top-left cell,
move only right or down to reach the bottom-right cell. Return the smallest
possible sum of the numbers along a path (both end cells included).

best[r][c] = grid[r][c] + min(best[r-1][c], best[r][c-1]) — each cell is reached
from above or from the left, so the cheapest path to it extends the cheaper of the
two.

Examples

solve([[1,3,1],[1,5,1],[4,2,1]])  → 7     // 1 → 3 → 1 → 1 → 1
solve([[1,2,3],[4,5,6]])          → 12
solve([[5]])                      → 5