返回排序二叉树第K小的数
还是用先序遍历，记录index和K进行比较
class Solution {
public:
void helper(TreeNode* node, int& idx, int k, int& res){
if(res!=INT_MAX)
return;
if(!node)
return;
//a(node)
//lk("root",node)
//dsp
helper(node->left, idx, k, res);
if(idx==k){
res=node->val;
//dsp
}
++idx;
helper(node->right, idx, k, res);
}
int kthSmallest(TreeNode* root, int k) {
int idx=;
int res=INT_MAX;
//ahd(root)
//a(res)
//a(k)
//a(idx)
helper(root, idx, k, res);
return res;
}
};