230.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 {
    int ans;
public:
    int kthSmallest(TreeNode* root, int &k) {
        if (!root) {
            return INT_MIN;
        }

        ans = kthSmallest(root->left, k);
        if (ans != INT_MIN) {
            return ans;
        }

        k--;
        if (k == 0) {
            return root->val;
        }

        ans = kthSmallest(root->right, k);

        return ans;
    }
};