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;
}