783 Minimum Distance Between BST Nodes

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.

Solution)

By using an inorder traverse, it can give a sorted list and calculate difference and find the minimum difference.

/**
 * 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) {
        // use inorder traverse
        Stack<TreeNode> stack = new Stack<>();
        TreeNode node = root;
        TreeNode prev = null;
        int minDiff = Integer.MAX_VALUE;
        while(!stack.isEmpty() || node != null) {
            while(node != null) {
                stack.push(node);
                node = node.left;
            }
            TreeNode temp = stack.pop();
            if (prev != null) {
                minDiff = Math.min(minDiff, temp.val - prev.val);
            }
            prev = temp;
            node = temp.right;
        }
        return minDiff;
    }
}