# LeetCode 530. Minimum Absolute Difference in BST

## Description:

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

## Note:

• There are at least two nodes in this BST.

## Example:

input:

   1
\
3
/
2


output:

output:

1

explanation:

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

## Solution:

/**
* Definition for a binary tree node.
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
int getMinimumDifference(TreeNode* root) {
if (root == NULL) {
return -1;
}

int minDiff = numeric_limits<int>::max();
findMinDiff(root, minDiff);

return minDiff;
}

void findMinDiff(TreeNode *root, int &minDiff) {

if(root->left != NULL){
recursiveFindDiff(root->left, root->val, minDiff);
findMinDiff(root->left, minDiff);
}

if (root->right != NULL) {
recursiveFindDiff(root->right, root->val, minDiff);
findMinDiff(root->right, minDiff);
}

}

void recursiveFindDiff(TreeNode *node, int comVal, int &minDiff) {
minDiff = min(minDiff, abs(node->val - comVal));

if (node->left != NULL) {
recursiveFindDiff(node->left, comVal, minDiff);
}

if (node->right != NULL) {
recursiveFindDiff(node->right, comVal, minDiff);
}

}

};


There are two layer of recursive passes in the algorithm. The first layer iterates through the entire binary tree; the second layer compute the minimum difference between the current node and the rest of the binary tree. The final result would be the minima of all differences.