(This post is part of the Genuinely Approachable Sudoku (GAS) series.)
Along each of the vertical bars in this cute Sudoku you must alternate numbers higher than their neighbor(s) with numbers lower than their neighbor(s). Can you figure out all the peaks and valleys to get to the one true solution?

PDF

or solve online (using SudokuPad)

Author/Opus: This is the 106th puzzle from Bill Murphy, part of the Genuinely Approachable Sudoku (GAS) team.

Rules: Standard Sudoku rules (insert a number from 1 to 9 into each cell so that no number repeats in any row, column, or bold region). Also, numbers must alternate between increasing and decreasing along the gray lines (e.g., a valid sequence for a gray line could be 2937186 with 9, 7, and 8 as high points and 2, 3, 1, and 6 as low points).

GAS Time Standards (highlight to view): Two party hats (🎩🥳): 8:30; One party hat (🥳): 16:00. All other solvers earn a 🦕: See-Saurornithoides.

Thomas Hits the GAS (highlight to view): 3:43, with SudokuPad replay file shared as a download for now (requires loading via settings menu with improvements expected before people should use this regularly).

Solution: PDF; solving video with explanation to be added later from GAS team.

Note: Follow this link for other less common variations of Sudoku.

Note 2: Comments on the blog are great! For a more interactive discussion, please also consider using our Daily GAS discussion post on the GMPuzzles Discord. Not a member of the Discord? Click this link for basic access.