[leetcode_230]Kth Smallest Element in a BST

求二叉搜索树的第k个数，中序遍历生成二叉搜索树，存入数组中，然后输出第k个即可。

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class Solution {
public:
    vector<int> list;
    void queryTree(TreeNode * node)
    {
        if (NULL == node)
        {
            return;
        }
        if (NULL != node->left)
        {
            queryTree(node->left);
        }
        list.push_back(node->val);
        if (NULL != node->right)
        {
            queryTree(node->right);
        }
    }

    int kthSmallest(TreeNode* root, int k) {
        list.clear();
        queryTree(root);
        return list[k-1];
    }
};