检查是否为BST

/*
struct TreeNode {
int val;
struct TreeNode *left;
struct TreeNode *right;
TreeNode(int x) :
val(x), left(NULL), right(NULL) {
}
};*/

class Checker {
public:
bool checkBST(TreeNode* root) {
return checkBST(root, INT_MIN, INT_MAX);
}
bool checkBST(TreeNode* root, int leftVal, int rightVal){
if(root->val > leftVal && root->val < rightVal){
if(root->left){
if(!checkBST(root->left, leftVal, root->val))
return false;
}
if(root->right){
if(!checkBST(root->right,root->val, rightVal))
return false;
}
return true;
}
return false;
}
};

TreeNode* pre = NULL;
bool checkBST(TreeNode* root) {
if(Inorder(root)!=NULL) return false;
return true;
}
TreeNode* Inorder(TreeNode *root) {
if(root==NULL) return NULL;
TreeNode* left=Inorder(root->left);
if(left!=NULL) return left;
if(pre!=NULL&&pre->val>root->val) return root;
pre = root;
TreeNode *right=Inorder(root->right);
if(right!=NULL) return right;
return NULL;
}