Eric 的小站

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输入：root = [3,1,4,null,2], k = 1
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输出：1

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输入：root = [5,3,6,2,4,null,null,1], k = 3
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输出：3

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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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    vector<int> vec;
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public:
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    int kthSmallest(TreeNode* root, int k) {
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        inOrder(root);
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        return vec[k-1];
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    }
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    void inOrder(TreeNode* root){
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        if(root == nullptr) return;
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        inOrder(root->left);
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        vec.push_back(root->val);
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        inOrder(root->right);
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    }
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};