Zigzag Conversion
mediumTime: O(n)Space: O(n)
Signals to notice
write string in zigzag across rowsread row by rowoscillating row index
Brute force first
Build a 2D grid — O(n × numRows) space.
The key insight
Array of strings (one per row). Walk through string assigning each char to its row. Row bounces: 0→numRows-1→0→... O(n).
What must stay true
The row index oscillates between 0 and numRows-1. Each character appends to its row's string. Concatenating all rows gives the result.
Easy way to go wrong
Edge case: numRows = 1 → return the original string. No zigzag needed.