530. 平衡二叉树中，相邻节点的最小差 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.

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.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int GetMinimumDifference(TreeNode root) {
		if (root == null) {
			return 0;
		}
		int min = Int32.MaxValue;
		Stack<TreeNode> stack = new Stack<TreeNode>();
		TreeNode current = root;
		while (current != null) {
			stack.Push(current);
			current = current.left;
		}
		while (stack.Count != 0) {
			current = stack.Pop();
			if (stack.Count > 0) {
				int diff = Math.Abs(stack.Peek().val - current.val);
				min = Math.Min(diff,min);
			}
			TreeNode node = current.right;
			while (node != null) {
				stack.Push(node);
				node = node.left;
				if (stack.Count > 0) {
					int diff = Math.Abs(stack.Peek().val - current.val);
					min = Math.Min(diff,min);
				}
			}
		}
		return min;
    }
}

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