Given the root of a complete binary tree, return the number of the nodes in the tree. According to Wikipedia, every level, except possibly the last, is completely filled in a complete binary tree, 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. Design an algorithm that runs in less than O(n) time complexity.

Example 1:

Input: root = [1,2,3,4,5,6]
Output: 6

Example 2:

Input: root = []
Output: 0

Example 3:

Input: root = [1]
Output: 1

-> 計算完整的樹節點

<解題>

class Solution {
    public int countNodes(TreeNode root) {
        if(root ==null) return 0;
        int ans = countAllNodes(root);
        return ans;
    }

    public int countAllNodes(TreeNode root){
        if(root ==null) return 0;
        int lh = countAllNodes(root.left);
        int rh = countAllNodes(root.right);

        return lh+rh+1;
    }
}

Time: O(n) Space: O(n)