Eric 的小站

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输入：root = [2,1,3]
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输出：true

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输入：root = [5,1,4,null,null,3,6]
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输出：false
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解释：根节点的值是 5 ，但是右子节点的值是 4 。

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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),
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 * right(right) {}
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 * };
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 */
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class Solution {
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    vector<long> vec;
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public:
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    bool isValidBST(TreeNode* root) {
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        inOrder(root);
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        for (int i = 1; i < vec.size(); i++) {
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            if (vec[i] - vec[i - 1] <= 0) {
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                return false;
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            }
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        }
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        return true;
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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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};