牛客网：NC84 完全二叉树结点数

 直接遍历

public class Solution {
    public int nodeNum(TreeNode head) {
        if (head == null){
            return 0;
        }
        return 1 + nodeNum(head.left) + nodeNum(head.right);
    }
}

使用满二叉树的性质

public class Solution {
    public int nodeNum(TreeNode head) {
        if (head == null){
            return 0;
        }
        int left = getHeight(head.left);
        int right = getHeight(head.right);
        if (left != right){
            return (int)Math.pow(2, right) + nodeNum(head.left); 
        } 
        return (int)Math.pow(2, left)  + nodeNum(head.right);
    }
    
    private int getHeight(TreeNode node){
        TreeNode cur = node;
        int res = 0;
        while (cur != null){
            cur = cur.left;
            res++;
        }
        return res;
    }
}