java: add «1530. Number of Good Leaf Nodes Pairs»

Signed-off-by: Matej Focko <me@mfocko.xyz>

java/number-of-good-leaf-nodes-pairs.java

@@ -0,0 +1,58 @@
+class Solution {
+  private record Result(int[] reachable, int pairs) {
+    public Result(int[] reachable) {
+      this(reachable, 0);
+    }
+
+    public static Result deadEnd(int distance) {
+      return new Result(new int[distance + 1]);
+    }
+
+    public static Result leaf(int distance) {
+      int[] reachable = new int[distance + 1];
+      reachable[0] = 1;
+      return new Result(reachable);
+    }
+
+    public static Result merge(int distance, Result left, Result right) {
+      int[] merged = new int[distance + 1];
+
+      // we're moving by one
+      for (int i = 0; i < distance; ++i) {
+        merged[i + 1] += left.reachable[i] + right.reachable[i];
+      }
+
+      int pairs = left.pairs + right.pairs;
+      for (int i = 0; i <= distance; ++i) {
+        for (int j = 0; j <= distance; ++j) {
+          if (i + j + 2 <= distance) {
+            pairs += left.reachable[i] * right.reachable[j];
+          }
+        }
+      }
+
+      return new Result(merged, pairs);
+    }
+  }
+
+  private boolean isLeaf(TreeNode node) {
+    return node.left == null && node.right == null;
+  }
+
+  private Result traverse(TreeNode node, int distance) {
+    if (node == null) {
+      return Result.deadEnd(distance);
+    } else if (isLeaf(node)) {
+      return Result.leaf(distance);
+    }
+
+    var left = traverse(node.left, distance);
+    var right = traverse(node.right, distance);
+
+    return Result.merge(distance, left, right);
+  }
+
+  public int countPairs(TreeNode root, int distance) {
+    return traverse(root, distance).pairs;
+  }
+}