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Strings — Zigzag Conversion

AlgorithmsStringsSimulation

solve(s, numRows) — write s in a zigzag pattern on numRows rows, going down then
diagonally up, then read the rows one after another. With "PAYPALISHIRING" on 3 rows:

P   A   H   N
A P L S I I G
Y   I   R

the answer is "PAHNAPLSIIGYIR". Do not draw the grid: keep one string per row, and move a
row index down until it hits the last row, then up until it hits row 0, flipping direction at
the edges. With a single row (or at least as many rows as characters) the string is unchanged.

Examples

solve("PAYPALISHIRING", 3)   → "PAHNAPLSIIGYIR"
solve("PAYPALISHIRING", 4)   → "PINALSIGYAHRPI"
solve("AB", 1)               → "AB"

Sample tests

Test #1Three rows
Input: ["PAYPALISHIRING",3]
Output: "PAHNAPLSIIGYIR"
Test #2Four rows
Input: ["PAYPALISHIRING",4]
Output: "PINALSIGYAHRPI"
Test #3Single character
Input: ["A",1]
Output: "A"
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