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.

Solution 1://先遍历左节点然后把左节点设为prev ，然后算差值，把自己设为prev然后遍历右节点，保证root永远大于prev

class Solution {

    int min = Integer.MAX_VALUE;

    Integer prev = null;

    public int getMinimumDifference(TreeNode root) {

        if (root == null)

            return min;

        getMinimumDifference(root.left);

        if (prev != null)

            min = Math.min(min, root.val - prev);

        prev = root.val;

        getMinimumDifference(root.right);

        return min;

    }

}

 

Solution 2://用先序遍历的方法

class Solution {

    int min = Integer.MAX_VALUE;

    TreeSet<Integer> set = new TreeSet<>();

    public int getMinimumDifference(TreeNode root) {

        if (root == null)

           return min;

        if (!set.isEmpty()) {

            if (set.floor(root.val) != null) //set.floor(val)找到set中等于val或小于val的最大的数

                min = Math.min(min, root.val - set.floor(root.val));

            if (set.ceiling(root.val) != null) //set.ceiling(val)找到set中等于val或大于val的最小的数

                min = Math.min(min, set.ceiling(root.val) - root.val); 

        }

        set.add(root.val);

        getMinimumDifference(root.left);

        getMinimumDifference(root.right);

        return min;

    }

}