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Question

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 $2^h$ nodes inclusive at the last level h.

Solution

Result: Accepted Time: 176 ms

Here should be some explanations.

class Solution {
public:
    int helper(TreeNode * root)
    {
        if(!root)
            return 0;
        int leftside=0, rightside=0;
        TreeNode *l=root, *r=root;
        while(l) leftside++,l=l->left;
        while(r) rightside++, r=r->right;
        if(rightside==leftside)
            return (1 << rightside)-1;
        return helper(root->left)+helper(root->right)+1;
    }
    int countNodes(TreeNode* root) {
        return helper(root);
    }
};

Complexity Analytics

• Time Complexity: $O(n)$
• Space Complexity: $O(1)$