# Count Complete Tree Nodes (Leetcode 222) - Top Google Coding Interview Question Mani
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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 2h nodes inclusive at the last level h.

Example:

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

Output: 6```

Solution:

``````1
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/**
* Time Complexity: O(N) where N is the number of nodes.
* Space Complexity: O(d) to keep recursion stack where d is the tree depth.
*/
class Solution {
public int countNodes(TreeNode root) {
dfs(root);
return count;
}

int count;

void dfs(TreeNode root) {
if(root == null) {
return;
}
count++;
dfs(root.left);
dfs(root.right);
}
}

/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode() {}
*     TreeNode(int val) { this.val = val; }
*     TreeNode(int val, TreeNode left, TreeNode right) {
*         this.val = val;
*         this.left = left;
*         this.right = right;
*     }
* }
*/
``````
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