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:

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.