Given a binary search tree with non-negative values, find the minimum absolute difference between values of any two nodes.
Example:

Input:

   1
    \
     3
    /
   2

Output:
1
Explanation:
The minimum absolute difference is 1, which is the difference between 2 and 1 (or between 2 and 3).

Note: There are at least two nodes in this BST.
BST中序遍歷是有序的。求最小值的時候就是看這個節(jié)點和前邊的節(jié)點的值是不是最小值就行了。
代碼如下：

/**
 * 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:
    int getMinimumDifference(TreeNode* root) {
        finMinValue(root);
        return r;
    }
    //中序遍歷
    void finMinValue(TreeNode* root)
    {
        if(root == NULL)
            return;
        finMinValue(root->left);
        if(pre != -1)
        {
            int v = abs(root->val - pre);
            if(v < r)
                r = v;
        }
        pre = root->val;
        finMinValue(root->right);
    }
private:
    int r = INT_MAX;
    int pre = -1;    
};