[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.

class Solution {
public:
bool isValidBST(TreeNode *root) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
if(root==NULL)
return true;
TreeNode *lm,*rm;
lm=leftmost(root);
rm=rightmost(root);
if(root->left!=NULL && root->left->val>=root->val || lm && lm->val >= root->val)
return false;
if(root->right!=NULL && root->right->val<=root->val || rm && rm->val<=root->val)
return false;
return isValidBST(root->left)&&isValidBST(root->right);
}
TreeNode* rightmost(TreeNode *root){
if(root->right)
root=root->right;
else
return NULL;
while(root->left)
root=root->left;
return root;
}
TreeNode* leftmost(TreeNode *root){
if(root->left)
root=root->left;
else
return NULL;
while(root->right)
root=root->right;
return root;
}
};