[LeetCode] 98. Validate Binary Search Tree

Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:

• The left subtree of a node contains only nodes with keys less than the node’s key.
• The right subtree of a node contains only nodes with keys greater than the node’s key.
• Both the left and right subtrees must also be binary search trees.

Example 1:

    2   / \  1   3

Binary tree [2,1,3], return true.

Example 2:

    1   / \  2   3

Binary tree [1,2,3], return false.

class Solution {public:    bool isValidBST(TreeNode* root) {        stack<TreeNode *> stk;        TreeNode *ptn = root;        bool oneshot = true;        int prev;        while (!stk.empty() || ptn) {            if (ptn) {                stk.push(ptn);                ptn = ptn->left;            } else {                ptn = stk.top();                stk.pop();                if (oneshot) {                    prev = ptn->val;                    oneshot = false;                } else if (ptn->val <= prev)                    return false;                prev = ptn->val;                ptn = ptn->right;            }        }        return true;    }};