CountCompleteTreeNodes.java

public class CountCompleteTreeNodes {
 /**
 *
 Given a complete binary tree, count the number of nodes.

 Note:

 Definition of a complete binary tree from Wikipedia:
 In a complete binary tree every level, except possibly the last, is completely filled, and all nodes in the last level are as far left as possible. It can have between 1 and 2h nodes inclusive at the last level h.

 Example:

 Input:
 1
 / \
 2 3
 / \ /
 4 5 6

 Output: 6
 */
 public int countNodes(TreeNode root) {
 if (root == null) return 0;
 int level = 0;

 TreeNode cur = root;
 while (cur != null) {
 level++;
 cur = cur.left;
 }

 int totalNodesBeforeLastLevel = (1 << (level - 1)) - 1;
 return totalNodesBeforeLastLevel + nodesOfLastLevel(root, level);
 }

 public int nodesOfLastLevel(TreeNode root, int level) {
 if (root == null) return 0;
 if (level == 1 && root != null) return 1;
 if (level == 2) {
 if (root.right != null) return 2;
 if (root.left != null) return 1;
 return 0;
 }
 TreeNode midNode = root.left;

 for (int i = 0; i < level - 2; i++) {
 midNode = midNode.right;
 }

 if (midNode == null) return nodesOfLastLevel(root.left, level - 1);
 return (1 << (level - 2)) + nodesOfLastLevel(root.right, level - 1);

 }

 public static void main(String[] args) {
 CountCompleteTreeNodes a = new CountCompleteTreeNodes();
 TreeNode b = new TreeNode(1);
 b.left = new TreeNode(2);
 b.right = new TreeNode(3);
 b.left.left = new TreeNode(4);
 b.left.right = new TreeNode(5);
 b.right.left = new TreeNode(6);

 System.out.println(a.countNodes(b));
 }
}