Trapping Rain Water

1. 1
   Learn the pattern

   Sorted input + pair/target question → two ends walking inward. Shrink from the side that can't work.

2. 2
   Trace your version

   Paste your solution into the workspace and watch every step execute.

   Open workspace
3. 3
   Solve it

   Solve it on LeetCode, then mark it solved here to keep your streak alive.

   Solve on LeetCode Mark solved