783. Minimum Distance Between BST Nodes

Given a Binary Search Tree (BST) with the root node root, return the minimum difference between the values of any two different nodes in the tree.

Example :

Input: root = [4,2,6,1,3,null,null]
Output: 1
Explanation:
Note that root is a TreeNode object, not an array.

The given tree [4,2,6,1,3,null,null] is represented by the following diagram:

          4
        /   \
      2      6
     / \    
    1   3  

while the minimum difference in this tree is 1, it occurs between node 1 and node 2, also between node 3 and node 2.

Note:

1. The size of the BST will be between 2 and 100.

2. The BST is always valid, each node's value is an integer, and each node's value is different.

// Inorder Traversal Recursion
void helper(TreeNode* node, TreeNode*& pre, int& res) {
    if (!node) return;
    helper(node->left, pre, res);
    if (pre)
        res = min(res, node->val - pre->val);
    pre = node;
    helper(node->right, pre, res);
}
int minDiffInBST(TreeNode* root) { // time: O(n); space: O(h)
    int res = INT_MAX;
    TreeNode* pre = nullptr;
    helper(root, pre, res);
    return res;
}