leetcode 530. Minimum Absolute Difference in BST

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

Example:

Input:

   1
    \
     3
    /
   2

Output:
1

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

题目大意，求bst树任意两点的最小绝对距离。

思路，中序遍历记录先前结点和当前结点的绝对距离，保存最小的。

/**
 * 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 = 1000000;
    void dfs(TreeNode* root, int& pre) {
        if (root == nullptr) return ;
        dfs(root->left, pre);
        if (pre != -1) ans = min(root->val-pre, ans);
        //cout << "pre " << pre << "    " << root->val << endl;
        pre = root->val;
        dfs(root->right, pre);
    }
    int getMinimumDifference(TreeNode* root) {
        int pre = -1;
        dfs(root, pre);
        return ans;
    }
};