题目：

题解：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */

int search_num(struct TreeNode* root, int k, int *result, int num)
{
    if(num == k + 1)
    {
        return k + 1; // 已找到就不往下跑了
    }

    if(root->left)
    {
        num = search_num(root->left, k, result, num);
    }
    if(num == k)
    {
        *result =  root->val;
        return k + 1; // 已找到就不往下跑了
    }
    num++;
    if(root->right)
    {
        num = search_num(root->right, k, result, num);
    }

    return num;
}

int kthSmallest(struct TreeNode* root, int k) {
    int result = 0;

    search_num(root, k, &result, 1); // 开始寻找

    return result;
}