java: add «437. Path Sum III»

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

java/path-sum-iii.java

@@ -0,0 +1,21 @@
+class Solution {
+  private int pathSumIncluding(TreeNode node, long target) {
+    if (node == null) {
+      return 0;
+    }
+
+    return (node.val == target ? 1 : 0)
+        + pathSumIncluding(node.left, target - node.val)
+        + pathSumIncluding(node.right, target - node.val);
+  }
+
+  public int pathSum(TreeNode node, int targetSum) {
+    if (node == null) {
+      return 0;
+    }
+
+    return pathSumIncluding(node, targetSum)
+        + pathSum(node.left, targetSum)
+        + pathSum(node.right, targetSum);
+  }
+}