题解 | #判断是不是二叉搜索树#

性质：二叉搜索树中序遍历是严格递增序列

做法：遍历时比较当前值和前一个值之间的大小关系即可

/**
 * struct TreeNode {
 *	int val;
 *	struct TreeNode *left;
 *	struct TreeNode *right;
 *	TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 * };
 */
class Solution {
public:
    bool isValidBST(TreeNode* root) {
      if (root == nullptr) {
        return true;
      }
      
      bool flag = true;
      int pre_num = 0;
      std::stack<TreeNode *> stack_;
      
      while (root || !stack_.empty()) {
        while (root) {
          stack_.push(root);
          root = root->left;
        }
        
        TreeNode *node = stack_.top();
        stack_.pop();
        
        if (flag) {
          flag = false;
        } else {
          if (pre_num >= node->val) {
            return false;
          }
        }
          
        pre_num = node->val;
        root = node->right;
      }
      
      return true;
    }
};