在Java中计算树中的节点

public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) { val = x; }
}

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