java: add «979. Distribute Coins in Binary Tree»

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

java/distribute-coins-in-binary-tree.java

@@ -0,0 +1,40 @@
+public class TreeNode {
+  int val;
+  TreeNode left;
+  TreeNode right;
+
+  TreeNode() {}
+
+  TreeNode(int val) {
+    this.val = val;
+  }
+
+  TreeNode(int val, TreeNode left, TreeNode right) {
+    this.val = val;
+    this.left = left;
+    this.right = right;
+  }
+}
+
+class Solution {
+  private int distribute(TreeNode node) {
+    if (node == null) {
+      return 0;
+    }
+
+    int left = distribute(node.left);
+    int right = distribute(node.right);
+
+    moves += Math.abs(left) + Math.abs(right);
+
+    return node.val - 1 + left + right;
+  }
+
+  private int moves;
+
+  public int distributeCoins(TreeNode root) {
+    moves = 0;
+    distribute(root);
+    return moves;
+  }
+}