力扣98. 验证二叉搜索树

深度优先遍历

• 思路：
  • 根据二叉搜索树特性，通过中序遍历得到有序序列，验证序列是否有序来判断；
  • 中序遍历使用栈通过深度优先遍历；

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    bool isValidBST(TreeNode* root) {
        std::stack<TreeNode*> stk;
        long long pre = (long long)(INT_MIN) - 1;

        TreeNode* node = root;
        while ((node != nullptr) || (!stk.empty())) {
            while (node != nullptr) {
                stk.push(node);
                node = node->left;
            }

            node = stk.top();
            stk.pop();
            if (node->val <= pre) {
                return false;
            }
            pre = node->val;
            node = node->right;
        }

        return true;
    }
};