Watering Plants

Simulate greedily watering plants left-to-right with a fixed-capacity can starting at the river: whenever remaining water can't fully cover the next plant you must walk back to the river and return, so accumulate per-plant walking plus the extra round-trip distance to refill. The challenge is computing the total steps efficiently by tracking remaining water and adding 2*(i+1) distance whenever a refill is needed (O(n) time, O(1) space).