Kth Smallest_Element_in_a_BST.cpp

//Solution 01:
class Solution {
public:
    vector<int> a;
    int kthSmallest(TreeNode* root, int k) {
        inorder(root);
        int ans = a[k-1];
        return ans;
    }
    void inorder(TreeNode* root){
        if(!root) return;
        inorder(root->left);
        a.push_back(root->val);
        inorder(root->right);
    }
};

//Solution 02:
class Solution {
public:
    int ans;
    
    int kthSmallest(TreeNode* root, int k) {
        inorder(root, k);
        return ans;
    }
    
    void inorder(TreeNode* root, int& k) {
        if (!root) return;
        inorder(root->left, k);
        if (--k == 0){
            ans = root->val;
            return;
        } 
        inorder(root->right, k);
    }  
};