Eric 的小站

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

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输入：root = []
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输出：[]

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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<TreeNode*> vec;
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public:
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    void flatten(TreeNode* root) {
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        preOrder(root);
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        TreeNode* p = root;
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        for(int i=1;i<vec.size();i++){
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            p->left = nullptr;
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            p->right = vec[i];
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            p = p->right;
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        }
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    }
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    void preOrder(TreeNode* root){
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        if(root == nullptr) return ;
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        vec.push_back(root);
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        preOrder(root->left);
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        preOrder(root->right);
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    }
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};