222. Count Complete Tree Nodes

• Difficulty: Medium

• Topics: Binary Search, Tree

• Similar Questions:

  • Closest Binary Search Tree Value

Problem:

Given a complete binary tree, count the number of nodes.

Note:

Definition of a complete binary tree from Wikipedia:
In a complete binary tree every level, except possibly the last, is completely filled, and all nodes in the last level are as far left as possible. It can have between 1 and 2^h nodes inclusive at the last level h.

Example:

Input: 
    1
   / \
  2   3
 / \  /
4  5 6

Output: 6

Solutions:

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

private:
    int getLeftLevel(TreeNode* root) {
        if (root == nullptr)    return 0;
        return 1 + getLeftLevel(root->left);
    }

    int getRightLevel(TreeNode* root) {
        if (root == nullptr)    return 0;
        return 1 + getRightLevel(root->right);
    }

};