Given a Binary Search Tree (BST) with the root node root, return the minimum difference between the values of any two different nodes in the tree.

Example :

Input: root = [4,2,6,1,3,null,null]
Output: 1
Explanation:
Note that root is a TreeNode object, not an array.

The given tree [4,2,6,1,3,null,null
 
] is represented by the following diagram:

          4
        /         2      6
     / \    
    1   3  

while the minimum difference in this tree is 1, it occurs between node 1 and node 2, also between node 3 and node 2.

找BST中任意兩節點間的最小差。

根據二叉搜索樹的特點，中序遍歷就是一組升序數組，最小差值必然在數組中相鄰元素中產生。

所以中序遍歷一遍二叉樹，返回相鄰遍歷的兩個點之間的最小差就好了。

class Solution {
private:
    int temp = INT_MAX;
    int res = INT_MAX;
public:
    int minDiffInBST(TreeNode* root) {
        if (root) {
            if (root->left)
                minDiffInBST(root->left);
            if (temp != INT_MAX)
                res = min(res, root->val - temp);
            temp 
 
= root->val;
            if (root->right)
                minDiffInBST(root->right);
        }
        return res;
    }
};

783. Minimum Distance Between BST Nodes