222. Count Complete Tree Nodes

문제

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.

https://leetcode.com/problems/count-complete-tree-nodes/

접근

#이진트리 #preorder #재귀

모든 노드의 수를 세어야 하므로 모든 노드를 순회하는 preorder 알고리즘으로 해결하였다. inorder, postorder로도 가능하다.

코드

/**
 * 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 {
public:
    int answer = 0;
    
    void preorder(TreeNode* node) {
        if(node == NULL) return;
            
        answer++;
        
        if(node->left != NULL) preorder(node->left);
        if(node->right != NULL) preorder(node->right);
    }
    
    int countNodes(TreeNode* root) {
        preorder(root);
        return answer;
    }
};

효율성

Runtime: 77 ms, faster than 18.19% of C++ online submissions for Count Complete Tree Nodes.
Memory Usage: 30.9 MB, less than 63.27% of C++ online submissions for Count Complete Tree Nodes.