783 Minimum Distance Between BST Nodes

783. Minimum Distance Between BST Nodes

1. Question

Given a Binary Search Tree (BST) with the root noderoot, 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.

Note:

1. The size of the BST will be between 2 and 100.

2. The BST is always valid, each node's value is an integer, and each node's value is different.

2. Implementation

(1) Inorder Traversal

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int minDiffInBST(TreeNode root) {
        int res = Integer.MAX_VALUE;

        TreeNode curNode = root, preNode = null;
        Stack<TreeNode> stack = new Stack<>();

        while (curNode != null || !stack.isEmpty()) {
            if (curNode != null) {
                stack.push(curNode);
                curNode = curNode.left;
            }
            else {
                curNode = stack.pop();
                if (preNode != null) {
                    res = Math.min(res, Math.abs(preNode.val - curNode.val));
                }
                preNode = curNode;
                curNode = curNode.right;
            }

        }
        return res;
    }
}

3. Time & Space Complexity

时间复杂度O(n), n为树的node的个数，空间复杂度O(h), h为树的高度