leetcode-530：二叉搜索树的最小绝对差

输入：
   1
    \
     3
    /
   2
输出：
1
解释：
最小绝对差为 1，其中 2 和 1 的差的绝对值为 1（或者 2 和 3）。

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def getMinimumDifference(self, root: TreeNode) -> int:
        stack = []
        cur = root
        prev = None
        best = float('inf')
        while cur or stack:
            while cur:
                stack.append(cur)
                cur = cur.left
            cur = stack.pop()
            if prev:
                best = min(best,abs(cur.val-prev.val))
            prev = cur
            cur  = cur.right
        return best

class Solution {
public:
    int getMinimumDifference(TreeNode* root) {
        int num=INT_MAX;
        stack<TreeNode*> stack;
        TreeNode* cur=root;
        TreeNode* prev=nullptr;
        while(!stack.empty()||cur){
            while(cur){
                stack.push(cur);
                cur=cur->left;
            }
            cur=stack.top();
            stack.pop();
            if(prev) num=min(num,abs(prev->val-cur->val));
            prev = cur;
            cur=cur->right;
        }
        return num;
    }
};