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程式語言 - LeetCode - C++ - 230. Kth Smallest Element in a BST

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題目：


解答：

/**
 * 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 {
public:
    int kthSmallest(TreeNode* root, int k) {
        auto dfs = [&](this auto&& dfs, TreeNode *n) -> int {
            if (!n) {
                return -1;
            }

            int l = dfs(n->left);

            if (k == 0) {
                return l;
            }

            k -= 1;

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

            return dfs(n->right);
        };

        return dfs(root);
    }
};