Pacific Atlantic Water Flow

An island is a grid of heights. The Pacific ocean touches the top and left edges; the Atlantic touches the bottom and right edges. Rain water flows from a cell to a neighbour (up/down/left/right) whose height is less than or equal, and from edge cells into the ocean next to them. Find every cell from which water can reach both oceans.

Input:

• line 1: r c
• next r lines: c heights each

Output:

• line 1: the number of cells k
• next k lines: row col (0-based), sorted by row, then by column

Constraints: 1 ≤ r, c ≤ 200, 0 ≤ height ≤ 10⁵.

Think about it: searching from every cell is slow. Reverse the flow instead: start from each ocean's edge cells and climb uphill (to neighbours with height ≥). The answer is the intersection of the two reachable sets.