isValidBST.cpp

/**
 * 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) {
        if (!root) return true;
        stack<TreeNode*> st;
        TreeNode *pre = nullptr;
        while (root || !st.empty()) {
            if (root) {
                st.push(root);
                root = root -> left;
            }
            else {
                root = st.top();
                st.pop();
                if (pre && root -> val <= pre -> val) return false;
                pre = root;
                root = root -> right;
            }
        }
        return true;
    }
};

class Solution {
public:
    long long pre = static_cast<long long>(INT_MIN) - 10LL;
    bool isValidBST(TreeNode* root) {
        if (!root) return true;
        if (!isValidBST(root->left))
            return false;
        if (root->val <= pre) return false;
        pre = root->val;
        return isValidBST(root->right);
    }
};