LeetCode Day 21 - Interactive Grid (Leftmost Column with 1)

Picked one by one from a mixed channel. Transcript check: both title readers certain; 5 of 5 sampled certain picks confirmed by transcript, none failed. Channel rule: Include tutorials and lectures, coding-interview and named-problem solution explanations, setup and testing how-tos, and Advent of Code and LeetCode problem walkthroughs. Exclude stream VODs, contest screencasts, and videos about his personal competition results.