[LeetCode] Validate Binary Search Tree

vector<int> inorderTraversal(TreeNode *root) {vector<TreeNode*> pre_node;vector<int> val;if (root == NULL){return val;}pre_node.push_back(root);while(!pre_node.empty()){TreeNode* cur_node = pre_node.back();TreeNode* left_leaf = NULL;TreeNode* right_leaf = NULL;if(cur_node != NULL){left_leaf = cur_node->left;right_leaf = cur_node->right;}if(left_leaf != NULL){pre_node.push_back(left_leaf);}else{val.push_back(cur_node->val);pre_node.pop_back();if(right_leaf != NULL){                pre_node.push_back(right_leaf);}else {while(!pre_node.empty()){cur_node = pre_node.back(); val.push_back(cur_node->val);pre_node.pop_back();if(cur_node->right != NULL){pre_node.push_back(cur_node->right);break;}                    }} }}return val;} bool isValidBST(TreeNode *root) {if (root == NULL){return true;}vector<int> inorderVals = inorderTraversal(root);int lastVal = inorderVals[0];for (int i = 1; i < inorderVals.size(); i++){if (inorderVals[i] <= lastVal){return false;}lastVal = inorderVals[i];}return true;}

先中序遍历，再判断遍历结果是否单调递增