530. Minimum Absolute Difference in BST笔记

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中序遍历是有序的。求最小值的时候就是看这个节点和前边的节点的值是不是最小值就行了。
代码如下：

/**
 * 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;
    }
    //中序遍历
    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;    
};