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/**
 * 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:
        set<int> se;
        void DFS(TreeNode* node){
            if(!node) return;   
            se.insert(node->val);
            DFS(node->left);  // Only follows leftmost path!
        }
    public:
        int kthSmallest(TreeNode* root, int k) {
            DFS(root);
            k--;
            return *se.begin()+k;  // Wrong! Adds k to first element
        }
};

prog.cpp:14:9: error: ‘set’ does not name a type
         set<int> se;
         ^~~
prog.cpp:15:18: error: ‘TreeNode’ has not been declared
         void DFS(TreeNode* node){
                  ^~~~~~~~
prog.cpp:21:25: error: ‘TreeNode’ has not been declared
         int kthSmallest(TreeNode* root, int k) {
                         ^~~~~~~~
prog.cpp: In member function ‘void Solution::DFS(int*)’:
prog.cpp:17:13: error: ‘se’ was not declared in this scope
             se.insert(node->val);
             ^~
prog.cpp:17:29: error: request for member ‘val’ in ‘* node’, which is of non-class type ‘int’
             se.insert(node->val);
                             ^~~
prog.cpp:18:23: error: request for member ‘left’ in ‘* node’, which is of non-class type ‘int’
             DFS(node->left);  // Only follows leftmost path!
                       ^~~~
prog.cpp: In member function ‘int Solution::kthSmallest(int*, int)’:
prog.cpp:24:21: error: ‘se’ was not declared in this scope
             return *se.begin()+k;  // Wrong! Adds k to first element
                     ^~

Standard output is empty