深度優先遍歷

• 思路：
  • 根據二叉搜索樹特性，通過中序遍歷得到有序序列，驗證序列是否有序來判斷；
  • 中序遍歷使用棧通過深度優先遍歷；

/*** 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;}
};