Binary search tree on a small element of K

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• Title Description

Given a binary search tree, write a function kthSmallest to find where the k-th smallest element.
Note:
You can assume that k is always active, 1 ≤ k ≤ number of binary search tree elements.

• Examples of a
  
• Example Two
  
• Code

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    //利用中序遍历即可得到从小到大的排列
    void order_travel(TreeNode* t, int& c, int& val, int k)
    {
        if(t==NULL)
            return;
        else
        {
            order_travel(t->left, c, val, k);
            c++;
            if(c==k)
            {
                val = t->val;
                return;
            }
            order_travel(t->right,c,val,k);
        }
    }
    int kthSmallest(TreeNode* root, int k) 
    {
        int val;        //表示最终值
        int c=0;          //表示次数
        order_travel(root, c, val, k);
        return val;   
    }
    
};