Eric 的小站

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输入：root = [1,2,3,4,5,6]
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输出：6

1
输入：root = []
2
输出：0

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/**
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 * Definition for a binary tree node.
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 * struct TreeNode {
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 *     int val;
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 *     TreeNode *left;
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 *     TreeNode *right;
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 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
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 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
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 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
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 * };
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 */
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class Solution {
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public:
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    int countNodes(TreeNode* root) {
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        if(root == nullptr) return 0;
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        return countNodes(root->left) + countNodes(root->right) + 1;
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    }
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};