Validate Binary Search Tree -- LeetCode

Pre-order

Display the data part of the root (or current node).

Traverse the left subtree by recursively calling the pre-order function.

Traverse the right subtree by recursively calling the pre-order function.

In-order

Traverse the left subtree by recursively calling the in-order function.

Display the data part of the root (or current node).

Traverse the right subtree by recursively calling the in-order function.

In a BST, in-order traversal retrieves data in sorted order.

Post-order

Traverse the left subtree by recursively calling the post-order function.

Traverse the right subtree by recursively calling the post-order function.

Display the data part of the root (or current node).

class Solution {
public:
bool validate(TreeNode* root, TreeNode* &pre)
{
if (root == NULL) return true;
if (!validate(root->left, pre)) return false;
if (pre != NULL && pre->val >= root->val) return false;
pre = root;
return validate(root->right, pre);
}
bool isValidBST(TreeNode* root) {
TreeNode* pre = NULL;
return validate(root, pre);
}
};