Interview Classic 150 Problem P98: Validate Binary Search Tree

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Timeline

Timeline

2025-10-31

init

Inorder traversal

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Problem:

P98 Validate Binary Search Tree

https://leetcode.cn/problems/validate-binary-search-tree/description/?envType=study-plan-v2&envId=top-interview-150

In-order traversal must be strictly increasing.

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)
	{
	}
};

#include <stack>
#include <climits>
using std::stack;

class Solution {
    public:
	bool isValidBST(TreeNode *root)
	{
		stack<TreeNode *> st;
		TreeNode *p = root, *left, *right;
		long long last = LONG_MIN;
		while (p || !st.empty()) {
			if (p) {
				st.push(p);
				p = p->left;
			} else {
				// visit
				p = st.top();
				st.pop();
				if (p->val <= last) {
					return false;
				}
				last = p->val;
				p = p->right;
			}
		}
		return true;
	}
};

leetcode hot 100 rewrite

#include <stack>
using std::stack;

class Solution {
    public:
        bool isValidBST(TreeNode *root)
        {
                // The in-order traversal sequence is sorted.
                stack<TreeNode *> stk;
                TreeNode *p = root, *last = nullptr;

                while (p || !stk.empty()) {
                        if (p) {
                                stk.push(p);
                                p = p->left;
                        } else {
                                p = stk.top();
                                stk.pop();
                                // visit p
                                if (last && p->val <= last->val)
                                        return false;

                                last = p;
                                p = p->right;
                        }
                }
                return true;
        }
};