题目：

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代码（首刷自解 2024年1月30日）：

class Solution {
public:int countNodes(TreeNode* root) {int res = 0;if (root == nullptr) return res;queue<TreeNode*> deque;TreeNode* cur = root;deque.push(cur);int size = 0;while (!deque.empty()) {size = deque.size();while (size--) {TreeNode* node = deque.front();deque.pop();res++;if (node->left) deque.push(node->left);if (node->right) deque.push(node->right);}}return res;}
};

代码 （首刷看解析 根据完全二叉树性质迭代）

class Solution {
public:int countNodes(TreeNode* root) {if (root == nullptr) return 0;TreeNode* left = root->left;TreeNode* right = root->right;int leftDepth = 0;int rightDepth = 0;while (left) {left = left->left;leftDepth++;}while (right) {right = right->right;rightDepth++;}if (leftDepth == rightDepth) {return (2 << leftDepth) - 1;}return countNodes(root->left) + countNodes(root->right) + 1;}
};