完全二叉树

// (https://leetcode-cn.com/problems/count-complete-tree-nodes/
class Solution {
    public int countNodes(TreeNode root) {
        TreeNode left = root, right = root;
        int leftCount = 0, rightCount = 0;
        while (left != null) {
            left = left.left;
            leftCount ++;
        }
        while (right != null) {
            right = right.right;
            rightCount ++;
        }
        if (leftCount == rightCount) {
            return (int)Math.pow(2, leftCount) - 1;
        }
        return 1 + countNodes(root.left) + countNodes(root.right);
    }
}