LeetCode 530. Minimum Absolute Difference in BST

原题链接在这里：https://leetcode.com/problems/minimum-absolute-difference-in-bst/

题目：

Given a binary search tree with non-negative values, find the minimum absolute difference between values of any two nodes.

Example:

Input:

   1
    \
     3
    /
   2

Output:
1

Explanation:
The minimum absolute difference is 1, which is the difference between 2 and 1 (or between 2 and 3).

Note: There are at least two nodes in this BST.

题解：

Binary Tree Inorder Traversal, 由于是BST, 所以是asending的, 取出最小difference.

Time Complexity: O(n). Space: O(logn), stack space.

AC Java:

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

如果不是BST的话可以借助于TreeSet<Integer> ts, 对于每一个node, 找出node.val在ts中的floor和ceil, 计算minimum difference. 再把node 本身的val加到ts中.

Time Complexity: O(nlogn). Space: O(n), ts size.

AC Java:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    int min = Integer.MAX_VALUE;
    TreeSet<Integer> ts = new TreeSet<Integer>();
    
    public int getMinimumDifference(TreeNode root) {
        if(root == null){
            return min;
        }
        
        if(!ts.isEmpty()){
            if(ts.floor(root.val) != null){
                min = Math.min(min, root.val-ts.floor(root.val));
            }
            if(ts.ceiling(root.val) != null){
                min = Math.min(min, ts.ceiling(root.val)-root.val);
            }
        }
        ts.add(root.val);
        
        getMinimumDifference(root.left);
        getMinimumDifference(root.right);
        return min;
    }
}

类似Validate Binary Search Tree, Find Mode in Binary Search Tree.