Leetcode 530. Minimum Absolute Difference in BST

教科书式编程：考察BST树中序有序。所以，我们仅仅需要对其中序记住前面一个节点，然后进行比较即可。最后更新ans

/**
 * 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: 
    TreeNode* pre = NULL; int ans = 999999;
    int getMinimumDifference(TreeNode* root) {
        inOrder(root);
        return ans;
    }
    void inOrder(TreeNode* root) {
        if(!root) return;
        inOrder(root->left);
        if(pre) ans = min(ans, abs(pre->val - root->val));
        pre = root;
        inOrder(root->right);
    }
};