[leetcode] Validate Binary Search Tree

Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:

• The left subtree of a node contains only nodes with keys less than the node’s key.
• The right subtree of a node contains only nodes with keys greater than the node’s key.
• Both the left and right subtrees must also be binary search trees.

confused what `"{1,#,2,3}"` means? > read more on how binary tree is serialized on OJ.

```//还好对于int32位长整形，涉及溢出问题时，我们还可以用long来覆盖。
//如果函数形参是long的话，恐怕就得用额外一个标志位表示溢出了。
/**
* Definition for a binary tree node.
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
bool isValidBST(TreeNode* root) {
if(root == NULL){
return true;
}
else{
return _isValidBST(root->left, LONG_MIN, root->val) && _isValidBST(root->right, (long)root->val + 1, LONG_MAX);
}
}
bool _isValidBST(TreeNode * root, long min, long max){
if(root == NULL){
return true;
}
if(root->val < min || root->val >= max){
return false;
}
else{
return _isValidBST(root->left, min, root->val) && _isValidBST(root->right, (long)root->val + 1, max);
}
}
};```

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