Validate Binary Search Tree

问题
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.

解答

bool inorderl(struct TreeNode *root,int a)
{
if(root==NULL) return true;
if(root->val>a || root->val==a)
return false;
return inorderl(root->left,a)&&inorderl(root->right,a);
}

bool inorderr(struct TreeNode *root,int a)
{
if(root==NULL) return true;
if(root->val<a || root->val==a)
return false;
return inorderr(root->left,a)&&inorderr(root->right,a);
}
bool isleft(struct TreeNode *root,int a)
{
if(root==NULL) return true;
if(root!=NULL && root->left==NULL) return true;

return inorderl(root->left,a);

}
bool isright(struct TreeNode *root,int b)
{
if(root==NULL) return true;
if(root!=NULL && root->right==NULL) return true;

return inorderr(root->right,b);
}
bool isValidBST(struct TreeNode* root)
{
if(root==NULL) return true;
if(!root->left && !root->right) return true;
if(isleft(root,root->val)==false || isright(root,root->val)==false)
return false;

if(root->left && root->right)
return isValidBST(root->left)&&isValidBST(root->right);
if(root->left && !root->right)
return isValidBST(root->left);
if(!root->left && root->right)
return isValidBST(root->right);

}