LeetCode | 530. 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.
題目鏈接

最容易想到的方法，由於是BST，直接中序遍歷後即可得到排序後的list，再把排序後的list兩兩比較找出相差最小的值即可。

/**
* Definition for a binary tree node.
* public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
    List<Integer> list = new ArrayList<Integer>();
    public int getMinimumDifference(TreeNode root) {
        traverse(root);
        int min = Integer.MAX_VALUE;
        for(int i = 0; i < list.size() - 1; i++) {
            int absDiffer = Math.abs(list.get(i) - list.get(i+1));
            min = absDiffer<min ? absDiffer : min;
        }
        return min;
    }

    private void traverse(TreeNode node) {
        if(node != null) {
            traverse(node.left);
            list.add(node.val);
            traverse(node.right);
        }
    }
}

由於BST本身子樹間就是有序的，即可以通過遞歸等效實現中序遍歷後兩兩比較的效果。代碼如下：

/**
* Definition for a binary tree node.
* public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
    private int min = Integer.MAX_VALUE;
    Integer prev = -1;
    public int getMinimumDifference(TreeNode root) {
        if(root == null) return min;

        getMinimumDifference(root.left);
        if(prev != -1) min = Math.min(min, Math.abs(prev - root.val));
        prev = root.val;

        getMinimumDifference(root.right);
        return min;
    }
}

而在discuss中有網友給出瞭如何實現非BST樹時如何得出最小差值。思路就是將非BST樹，利用API中的TreeSet將其轉化爲BST樹再來進行兩兩比較，本質上和上面寫法是一樣的。

public class Solution {
    TreeSet<Integer> set = new TreeSet<>();
    int min = Integer.MAX_VALUE;

    public int getMinimumDifference(TreeNode root) {
        if (root == null) return min;

        if (!set.isEmpty()) {
            if (set.floor(root.val) != null) {
                min = Math.min(min, Math.abs(root.val - set.floor(root.val)));
            }
            if (set.ceiling(root.val) != null) {
                min = Math.min(min, Math.abs(root.val - set.ceiling(root.val)));
            }
        }

        set.add(root.val);

        getMinimumDifference(root.left);
        getMinimumDifference(root.right);

        return min;
    }
}