Two-Pointers

LeetCode 125 note: two pointers skip non-alphanumeric characters and compare lowercase letters, with digits included

LeetCode 28 note: try matching needle at every start, or use str.find; index raises when not found

LeetCode 27 note: the write pointer copies only values not equal to val, returning how many were written

LeetCode 26 note: overwrite in place with a write pointer for the next slot, writing only values different from the last one written