tips: 二叉树的中序遍历
判断一个二叉树是不是二叉搜索树,可以通过下面两种方式:
跟节点取值范围不限,左子树取值为小于左边界大于跟节点,右子树取值为大于跟节点小于右边界
)/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
bool help(TreeNode* root,long long leftMin, long long rightMax) {
if(root ==nullptr) {
return true;
}
if (root->val <= leftMin || root->val >= rightMax) {
return false;
}
return help(root->left,leftMin, root->val) && help(root->right,root->val,rightMax);
}
bool isValidBST(TreeNode* root) {
return help(root,LONG_MIN, LONG_MAX);
}
};
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
void addTreeNode(vector<int> &treeVector, TreeNode* root) {
if (root == NULL) {
return ;
}
addTreeNode(treeVector, root->left);
treeVector.push_back(root->val);
addTreeNode(treeVector, root->right);
}
bool isValidBST(TreeNode* root) {
if (root == nullptr) {
return true;
}
vector<int> treeVector;
addTreeNode(treeVector, root);
int size = treeVector.size();
if (size == 0) {
return true;
}
for(int i=1; i < size; i++) {
if (treeVector[i] <= treeVector[i-1]) {
return false;
}
}
return true;
}
};
END!