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 keysless 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.
代码如下：

bool isValidBST(TreeNode *root) {
if(root==NULL) return true;
stack<TreeNode*> stk;
TreeNode *p=root,*r=NULL;//r代表当前节点p的前一个节点
while(p || !stk.empty())
{
if(p)
{
stk.push(p);
p=p->left;
}
else
{
p=stk.top();
stk.pop();
if(r && r->val >=p->val)
return false;
r=p;
p=p->right;
}
}
return true;
}