Validate Binary Search Tree
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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.
题意：给定一棵二叉树，推断它是不是合法的二叉查找树
思路：dfs
合法二叉查找树必须满足下面两个条件
1.左子树和右子树都是合法二叉查找树
2.左子树的最右叶子节点 < 根 < 右子树的最左叶子节点
复杂度：时间O(n)，空间O(log n)
bool isValidBST(TreeNode *root) {
if(!root) return true;
TreeNode *right_most = root->left, *left_most = root->right;
while(right_most && right_most->right){
right_most = right_most->right;
}
while(left_most && left_most->left){
left_most = left_most->left;
}
return isValidBST(root->left) && isValidBST(root->right)
&& (!right_most || right_most->val < root->val)
&& (!left_most || root->val < left_most->val);
}