【LeetCode 530】 Minimum Absolute Difference in BST

題目描述

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.
This question is the same as 783: https://leetcode.com/problems/minimum-distance-between-bst-nodes.

思路

BST先序遍歷序列是升序的。

代碼

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int getMinimumDifference(TreeNode* root) {
        stack<TreeNode*> st;
        int pre = INT_MAX;
        int res = INT_MAX;
        while(root != NULL || !st.empty()) {
            while (root) {
                st.push(root);
                root = root->left;
            }
            root = st.top();
            st.pop();
            if (pre == INT_MAX) pre = root->val;
            else {
                res = min(res, root->val - pre);
                pre = root->val;
            }
            root = root->right;
        }
        return res;
    }
};