Leetcode#98 Validate Binary Search Tree

原题地址

 

中序遍历二叉树，如果出现逆序对，则说明不是合法BST

 

代码：

bool isValidBST(TreeNode *root) {
        stack<TreeNode *> st;
        int last = 0;
        bool hasLast = false;
        TreeNode *node = NULL;
        
        node = root;
        while (node) {
            while (node) {
                st.push(node);
                node = node->left;
            }
            node = NULL;
            while (!st.empty() && !node) {
                if (hasLast && st.top()->val <= last)
                    return false;
                hasLast = true;
                last = st.top()->val;
                node = st.top()->right;
                st.pop();
            }
        }
        
        return true;
}