[LeetCode] 222. Count Complete Tree Nodes

Problem

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

Solution

class Solution {
    public int countNodes(TreeNode root) {
        if (root == null) return 0;
        int height = 0;
        TreeNode left = root.left, right = root.right;
        while (left != null && right != null) {
            height++;
            left = left.left;
            right = right.right;
        }
        if (left == null && right == null) return (1 << (height+1)) - 1;
        else return 1 + countNodes(root.left) + countNodes(root.right);
    }
}
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