Second-Minimum-Node-In-A-Binary-Tree

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题目地址

LeetCode#671 Second Minimum Node In A Binary Tree

题目描述

  Given a non-empty special binary tree consisting of nodes with the non-negative value, where each node in this tree has exactly twoor zero sub-node. If the node has two sub-nodes, then this node’s value is the smaller value among its two sub-nodes.

  Given such a binary tree, you need to output the second minimum value in the set made of all the nodes’ value in the whole tree.

  If no such second minimum value exists, output -1 instead.

Example 1:

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Input: 
    2
   / \
  2   5
     / \
    5   7

Output: 5
Explanation: The smallest value is 2, the second smallest value is 5.

Example 2:

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Input: 
    2
   / \
  2   2

Output: -1
Explanation: The smallest value is 2, but there isn't any second smallest value.

解题思路

  题目要求得到二叉树中第二小的节点值 (Second Minimum Node) ，遍历就可以了。

解题代码

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/**
 * 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 {
    int fmin = INT_MAX;
    int smin = INT_MAX;
public:
    int findSecondMinimumValue(TreeNode* root) {
        if (!root) return smin;
        int val = root->val;
        if (val < fmin) {
            smin = fmin;
            fmin = val;
        } else if (val > fmin && val < smin){
            smin = val;
        }
        findSecondMinimumValue(root->left);
        findSecondMinimumValue(root->right);
        return smin == INT_MAX ? -1 : smin;
    }
};
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