Minimum Absolute Difference in BST

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

Note: There are at least two nodes in this BST.

递归的解法中需要特别注意 min 和 prev 的设定，作为参数传入均不太妥当。由于二叉搜索树的特性，求得最小差值时无需判断绝对值。