783. Minimum Distance Between BST Nodes

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.

/**
 * 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) {
        Queue<TreeNode> queue = new LinkedList<>();
        List<Integer> list = new ArrayList<>();
        
        queue.offer(root);
        while(queue.size() > 0){
            int size = queue.size();
            for(int i=0; i<size; i++){
                TreeNode node = queue.poll();
                if(node.left != null){
                    queue.offer(node.left);
                }
                if(node.right != null){
                    queue.offer(node.right);
                }
                
                list.add(node.val);
            }
            
        }
        
        Collections.sort(list);
        
        int minDiff = Integer.MAX_VALUE;
        int prev = list.get(0);
        for(int i=1; i<list.size(); i++){
            int tmpDiff = Math.abs(prev - list.get(i));
            if(tmpDiff < minDiff){
                minDiff = tmpDiff;
            }
            prev = list.get(i);
        }
        
        return minDiff;
    }
}

/**
 * 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) {
        Queue<TreeNode> queue = new LinkedList<>();
        List<Integer> list = new ArrayList<>();
        
        traverseBST(root, list);
        
        int minDiff = Integer.MAX_VALUE;
        int prev = list.get(0);
        for(int i=1; i<list.size(); i++){
            int tmpDiff = Math.abs(prev - list.get(i));
            if(tmpDiff < minDiff){
                minDiff = tmpDiff;
            }
            prev = list.get(i);
        }
        
        return minDiff;
    }
    
    public void traverseBST(TreeNode node, List<Integer> list){
        if(node == null){
            return;
        }
        
        traverseBST(node.left, list);
        list.add(node.val);
        traverseBST(node.right, list);
    }
}

[문제] https://leetcode.com/problems/minimum-distance-between-bst-nodes/