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copied!<p>Looks like <code>PAYPALISTHEFASTERSAFERWAYTOSENDMONEY</code> is the input and </p> <pre><code>P A Y P A L F E R W A I A M O N Y S S D Y E T T R N E S O H E T S A F E </code></pre> <p>is the output to me..</p> <p>Even though the question was not clearly stating to provide an algorithm initially. Here is pseudo code for the recursive solution:</p> <pre><code>convert(input): spiral(out[][],input,0,0,sqrt(input.len)) return out.toString() spiral(out[][],input,ix,iy,size) if size>0 //calculate the frame coords with starting indices ix,iy & size of the frame place first 4*(size-1) chars on a frame on the ´out´ matrix //recursive call to create inner frame spiral(out,input.remainingString(),ix+1,iy+1,size-2) else return </code></pre> <p>and implementation in java:</p> <pre><code>public class PayPal { private enum Dir { RIGHT, DOWN, LEFT, UP; } public String convert(String input) { double dRoot = Math.sqrt(input.length()); int root; if (Double.compare(dRoot, (int) dRoot) == 0) { root = (int) dRoot; } else { root = (int) dRoot + 1; } char[][] out = new char[root][root]; spiral(out, 0, 0, root, input); StringBuilder sb = new StringBuilder(); for (char[] line : out) { sb.append(line); } return sb.toString(); } private void spiral(char[][] out, int i, int j, int size, String input) { Dir direction = Dir.RIGHT; if (size > 0) { if (size == 1) { out[i][j] = input.charAt(0); } else { for (int k = 0; k < 4 * (size - 1); k++) { int di = (k != 0 && k % (size - 1) == 0 ? size - 1 : k % (size - 1)); switch (direction) { case RIGHT: out[i][j + di] = input.charAt(k); break; case DOWN: out[i + di][j + size - 1] = input.charAt(k); break; case LEFT: out[i + size - 1][j + size - 1 - di] = input.charAt(k); break; case UP: out[i + size - 1 - di][j] = input.charAt(k); break; } if (k != 0 && (k % (size - 1) == 0)) //Change direction { direction = Dir.values()[direction.ordinal() + 1]; } } } spiral(out, i + 1, j + 1, size - 2, input.substring(4 * (size - 1))); } else { return; } } } </code></pre>

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