LeetCode Minimum Absolute Difference in BST

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

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;
     }
 }