【LeetCode】222. Count Complete Tree Nodes

Count Complete Tree Nodes

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

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.

完全二叉树相比普通二叉树，在计数时候一个trick在于，当最后一层是满的（最左深度等于最右深度），

就不需要通过遍历方式来计数，直接等于2^h - 1

/**
* 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 == NULL)
return 0;
TreeNode* l = root;
int lefth = 0;
while(l->left)
{
lefth ++;
l = l->left;
}
TreeNode* r = root;
int righth = 0;
while(r->right)
{
righth ++;
r = r->right;
}
if(lefth == righth)
return pow(2.0, lefth+1) - 1;
else
return 1 + countNodes(root->left) + countNodes(root->right);
}
};