
中序遍历,第k次出现的数值就是结果
cpp
/**
* 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 {
private:
int dfs(TreeNode* root,int& k){
if(root==NULL)
return -1;
else{
int result=dfs(root->left,k);
if(result!=-1)
return result;
if(--k==0)
return root->val;
return dfs(root->right,k);
}
}
public:
int kthSmallest(TreeNode* root, int k) {
return dfs(root,k);
}
};