leetcode 530. Minimum Absolute Difference in BST

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).

Note: There are at least two nodes in this BST.
利用中序遍历，前后对比

/**
* 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)
{
stack<TreeNode *> s;
TreeNode * p = root;
int ret = INT_MAX;
int last = -1;
while (p != NULL || !s.empty())
{
while ( p != NULL)
{
s.push(p);
p = p->left;
}
if (!s.empty()) //来到了最左边
{
p = s.top();
if (last != -1)
ret = min(ret, p->val - last);
last = p->val;
s.pop();
p = p->right;
}
}
return ret;
}
};