222. Count Complete Tree Nodes

• Recursion

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int countNodes(TreeNode* root)
    {
        int cnt = 0;
        if (!root) return 0;
        return 1 + countNodes(root->left) + countNodes(root->right);
    }
};

• T: $O(n)$
• S: $O(n)$

2

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int countNodes(TreeNode* root)
    {
        int leftHeight = 0, rightHeight = 0;

        TreeNode* leftNode = root;
        TreeNode* rightNode = root;

        while (leftNode)
        {
            ++leftHeight;
            leftNode = leftNode->left;
        }

        while (rightNode) {
            ++rightHeight;
            rightNode = rightNode->right;
        }

        if (leftHeight == rightHeight)
        {
            return pow(2, leftHeight) - 1;
        }

        return countNodes(root->left) + countNodes(root->right) + 1;
    }
};

• T: $O(\log n \cdot \log n)$
• S: $O(n)$