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.
1 bool varifyBST(TreeNode * root, int * out_min, int * out_max) { 2 if(root == NULL) 3 return true; 4 (*out_min) = root->val; 5 (*out_max) = root->val; 6 if(root->left == NULL && root->right == NULL) 7 return true; 8 9 int vmax = root->val; 10 int vmin = root->val; 11 if(root->left != NULL){ 12 bool ret = varifyBST(root->left, &vmin, &vmax); 13 if(ret == false) 14 return false; 15 if(vmax >= root->val) 16 return false; 17 (*out_min) = min(vmin, root->val); 18 } 19 20 if(root->right != NULL){ 21 bool ret = varifyBST(root->right, &vmin, &vmax); 22 if(ret == false) 23 return false; 24 if(vmin <= root->val) 25 return false; 26 (*out_max) = max(vmax, root->val); 27 } 28 return true; 29 } 30 31 bool isValidBST(TreeNode *root) { 32 int vmax = 0; 33 int vmin = 0; 34 return varifyBST(root, &vmin, &vmax); 35 }