LeetCode OJ 222. Count Complete Tree Nodes

Posted 2020-08-19

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.

 

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解答

这道题目因为是完全二叉树，所以求节点数时可以做一下优化，当恰好全部充满时直接计算节点数，而不是继续用递归……否则有个点会过不去……

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
int countNodes(struct TreeNode* root) {
    int level = 0;
    struct TreeNode *node_1 = root, *node_2 = root;
    
    while(NULL != node_1&&NULL != node_2){
        node_1 = node_1->left;
        node_2 = node_2->right;
        level++;
    }
    if(NULL != node_1){
        return countNodes(root->left) + countNodes(root->right) + 1;
    }
    else{
        return pow(2, level) - 1;
    }
}