LeetCode题解： Validate Binary Search Tree

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.

思路：

仍然是树的深度优先搜索。这里要注意的是，不能仅仅和父结点的值比较，还必须和上面所有的结点进行比较。

题解：

/**
* Definition for binary tree
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
vector<pair<int, char>> history;

bool isValidBST(TreeNode *node) {
if (node == nullptr)
return true;

// check if I am ok with all the parents
for(auto& h : history)
if (h.second == 'R')
{
// i am at the right subtree
if (node->val <= h.first)
return false;
}
else
{
// i am at the left subtree
if (node->val >= h.first)
return false;
}

bool leftok = true;
history.push_back(make_pair(node->val, 'L'));
leftok = isValidBST(node->left);

bool rightok = true;
history.back().second = 'R';
rightok = isValidBST(node->right);

history.pop_back();

return leftok && rightok;
}
};