题解 | #判断是不是二叉搜索树#

/**
 * struct TreeNode {
 *  int val;
 *  struct TreeNode *left;
 *  struct TreeNode *right;
 *  TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 * };
 */
class Solution {
  public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param root TreeNode类
     * @return bool布尔型
     */
    bool fl = true;
    int dfs(TreeNode* root) {
        if (root == nullptr)return 0xcccc;
        int le = root->val, ri = root->val;
        if (root->left != nullptr)le = dfs(root->left);
        if (root->right != nullptr)ri = dfs(root->right);
        if (root->val < le)fl = false;
        if (root->val > ri)fl = false;
        return max(max(le, ri), root->val);
    }
    bool isValidBST(TreeNode* root) {
        dfs(root);
        return fl;
    }
};