Kth Smallest Element In Tree

• Time : $O(nlogn)$
• Space : $O(n)$

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
void dfs(TreeNode* A, vector<int>& now) {
    if(A == NULL) return;
    now.push_back(A->val);
    dfs(A->left, now);
    dfs(A->right, now);
}
int Solution::kthsmallest(TreeNode* A, int B) {
    vector<int> now;
    dfs(A,now);
    sort(begin(now),end(now));
    return now[B-1];
}