1261. Find Elements in a Contaminated Binary Tree**

1261. Find Elements in a Contaminated Binary Tree**

https://leetcode.com/problems/find-elements-in-a-contaminated-binary-tree/

题目描述

Given a binary tree with the following rules:

1. root.val == 0
2. If treeNode.val == x and treeNode.left != null, then treeNode.left.val == 2 * x + 1
3. If treeNode.val == x and treeNode.right != null, then treeNode.right.val == 2 * x + 2

Now the binary tree is contaminated, which means all treeNode.val have been changed to -1.

You need to first recover the binary tree and then implement the FindElements class:

• FindElements(TreeNode* root) Initializes the object with a contamined binary tree, you need to recover it first.
• bool find(int target) Return if the target value exists in the recovered binary tree.

Example 1:

Input
["FindElements","find","find"]
[[[-1,null,-1]],[1],[2]]
Output
[null,false,true]
Explanation
FindElements findElements = new FindElements([-1,null,-1]); 
findElements.find(1); // return False 
findElements.find(2); // return True 

Example 2:

Input
["FindElements","find","find","find"]
[[[-1,-1,-1,-1,-1]],[1],[3],[5]]
Output
[null,true,true,false]
Explanation
FindElements findElements = new FindElements([-1,-1,-1,-1,-1]);
findElements.find(1); // return True
findElements.find(3); // return True
findElements.find(5); // return False

Example 3:

Input
["FindElements","find","find","find","find"]
[[[-1,null,-1,-1,null,-1]],[2],[3],[4],[5]]
Output
[null,true,false,false,true]
Explanation
FindElements findElements = new FindElements([-1,null,-1,-1,null,-1]);
findElements.find(2); // return True
findElements.find(3); // return False
findElements.find(4); // return False
findElements.find(5); // return True

Constraints:

• TreeNode.val == -1
• The height of the binary tree is less than or equal to 20
• The total number of nodes is between [1, 10^4]
• Total calls of find() is between [1, 10^4]
• 0 <= target <= 10^6

C++ 实现 1

本题是说, 一棵二叉树原本满足一开始的三个条件, 但是现在收到了污染, 所有节点的值都变成了 -1, 因此需要先编写方法恢复出原来的二叉树, 然后在恢复出来的二叉树中查找指定元素. 可以采用哈希表保存元素, 查找元素就非常方便. 而恢复二叉树的工作, 可以使用前序遍历.

class FindElements {
private:
    unordered_set<int> record;
    void recover(TreeNode *root) {
        if (!root) return;
        if (root->left) {
            root->left->val = 2 * root->val + 1;
            record.insert(root->left->val);
        }
        if (root->right) {
            root->right->val = 2 * root->val + 2;
            record.insert(root->right->val);
        }
        recover(root->left);
        recover(root->right);
    }
public:
    FindElements(TreeNode* root) {
        if (root) {
            root->val = 0;
            record.insert(root->val);
            recover(root);
        }
    }
    
    bool find(int target) {
        return record.count(target);
    }
};

/**
 * Your FindElements object will be instantiated and called as such:
 * FindElements* obj = new FindElements(root);
 * bool param_1 = obj->find(target);
 */