222_Count_Complete_Tree_Nodes.cpp

/*
222. Count Complete Tree Nodes
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
Constraints:
The number of nodes in the tree is in the range [0, 5 * 104].
0 <= Node.val <= 5 * 104
The tree is guaranteed to be complete.
*/
/**
 * 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 {
private:
void solve(TreeNode* root,int &count){
    if(!root) return;
    if(!root->right || root->left) count++;
    
    solve(root->left,count);
    solve(root->right,count);
}
public:
    int countNodes(TreeNode* root) {
        int count = 0 ;
        solve(root,count);
        return count ;
    }
};