Zigzag Conversion

Simulate the zigzag path rather than drawing a matrix. If numRows is one or at least the string length, return the string immediately. Otherwise create one builder per row, begin at row zero moving down, append each character to the current row, and reverse direction when reaching row zero or row numRows-1. Concatenate builders in order. Every character is appended once, so time is O(n) and space is O(n) for the output plus O(numRows) builders. An arithmetic cycle solution is possible, but simulation is clearer and less error-prone in an interview. Test empty string, one row, two rows, a length shorter than rows, and boundary characters at each direction reversal. State that indexing uses zero-based row positions even if the prompt names rows one-based.