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LeetCode OJ - Validate Binary Search Tree
题目:
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.
解题思路:
中序遍历二叉树,设置pre指针,在遍历过程中记录是否发生违反BST的节点的存在。
代码:
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 13 TreeNode * pre = NULL; 14 bool isValidBST(TreeNode *root) { 15 if (root == NULL) return true; 16 17 if (root->left != NULL && !isValidBST(root->left)) return false; 18 if (pre != NULL && pre->val >= root->val) return false; 19 pre = root; 20 if (root->right != NULL && !isValidBST(root->right)) return false; 21 return true; 22 } 23 };
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