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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.
confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.
中序遍历这棵树，如果前驱大于等于当前节点，那么就不是bst树
1 /** 2 * Definition for binary tree 3 * struct TreeNode { 4 * int val; 5 * TreeNode *left; 6 * TreeNode *right; 7 * TreeNode(int x) : val(x), left(NULL), right(NULL) {} 8 * }; 9 */10 class Solution {11 public:12 bool isValidBST(TreeNode *root) {13 if( !root ) return true;14 stack<TreeNode*> st;15 TreeNode* cur = root;16 while( cur ) {17 st.push(cur);18 cur = cur->left;19 }20 TreeNode* pre = 0;21 while( !st.empty() ) {22 cur = st.top();23 st.pop();24 if( pre && pre->val >= cur->val ) return false;25 pre = cur;26 cur = cur->right;27 while( cur ) {28 st.push(cur);29 cur = cur->left;30 }31 }32 return true;33 }34 };
Validate Binary Search Tree
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