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;
    }
};