LeetCode 530. Minimum Absolute Difference in BST(在二叉查找树中查找两个节点之差的最小绝对值)

题意：在二叉查找树中查找两个节点之差的最小绝对值

/**
 * 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 ans = 0x3f3f3f3f;
    TreeNode* pre = NULL;
    void inorder(TreeNode* root){
        if(root == NULL) return;
        inorder(root -> left);
        if(pre) ans = min(ans, root -> val - pre -> val);
        pre = root;
        inorder(root -> right);
    }
    int getMinimumDifference(TreeNode* root) {
        inorder(root);
        return ans;
    }
};