Create ZigZagConversion.java

java/ZigZagConversion.java

@@ -0,0 +1,74 @@
+// https://leetcode.com/problems/zigzag-conversion/
+public final class ZigZagConversion {
+    enum Direction {
+        SOUTH,
+        DIAGONAL
+    }
+
+    /**
+     * Apply zigzag conversion:
+     * PAYPALISHIRING -> PAHNAPLSIIGYIR
+     *
+     * Here are the things I'm thinking of.
+     *
+     * numRows = column size
+     * numRows-2 = diagonal size
+     *
+     * So we want to understand how long the string is. Then we want
+     * to go as follows:
+     *
+     * - Grab numRows chars
+     * - Append to a mapping where key = rawIndex, value = list of chars
+     * - Grab numRows-2
+     * - Append to a mapping where key = rawIndex-1, value = list of chars
+     * - If at any point we run out of characters, we stop the iteration
+     */
+    public String convert(String s, int numRows) {
+        StringBuilder result = new StringBuilder();
+
+        // Initialize the mapping
+        StringBuilder[] allRows = new StringBuilder[numRows];
+        for (int i = 0; i < numRows; i++) {
+            allRows[i] = new StringBuilder();
+        }
+
+        // Build the mapping
+        helper(s, numRows, allRows, Direction.SOUTH, 0);
+
+        // Join the mapping strings together
+        for (int depth = 0; depth < numRows; depth++) {
+            result.append(allRows[depth].toString());
+        }
+
+        return result.toString();
+    }
+
+    public void helper(
+        String s,
+        int numRows,
+        StringBuilder[] allRows,
+        Direction direction,
+        int absoluteIndex
+    ) {
+        int iterations = (direction == Direction.SOUTH)
+            ? numRows : numRows - 2;
+
+        for (int i = 0; i < iterations; i++) {
+            if (absoluteIndex == s.length()) {
+                return;
+            }
+
+            int depth = (direction == Direction.SOUTH)
+                ? i : iterations - i;
+
+            allRows[depth].append(s.charAt(absoluteIndex));
+
+            absoluteIndex++;
+        }
+
+        Direction nextDirection = (direction == Direction.SOUTH)
+            ? Direction.DIAGONAL : Direction.SOUTH;
+
+        helper(s, numRows, allRows, nextDirection, absoluteIndex);
+    }
+}