完全二叉树的节点个数

class Solution {
public:
    int countNodes(TreeNode* root) {
        if (root == nullptr) return 0;
        TreeNode* left = root->left;
        TreeNode* right = root->right;
        int leftDepth = 0, rightDepth = 0; // 这里初始为0是有目的的，为了下面求指数方便
        while (left) {  // 求左子树深度
            left = left->left;
            leftDepth++;
        }
        while (right) { // 求右子树深度
            right = right->right;
            rightDepth++;
        }
        if (leftDepth == rightDepth) {
            return (2 << leftDepth) - 1; // 注意(2<<1) 相当于2^2，所以leftDepth初始为0
        }
        return countNodes(root->left) + countNodes(root->right) + 1;
    }
};