notes/OJ notes/pages/Leetcode Two-Sum-IV-Input-Is-a-BST.md

Leetcode Two-Sum-IV-Input-Is-a-BST

2022-07-08 11:11

Algorithms:

#algorithm #binary_search #BFS

Data structures:

#DS #binary_tree #binary_search_tree

Difficulty:

#coding_problem #difficulty_easy

Additional tags:

#leetcode

Revisions:

N/A

Related topics:

Links:

• Link to problem
• Three method to solve this

---

Problem

Given the root of a Binary Search Tree and a target number k, return true if there exist two elements in the BST such that their sum is equal to the given target.

Examples

Example 1:

Input: root = [5,3,6,2,4,null,7], k = 9 Output: true

Example 2:

Input: root = [5,3,6,2,4,null,7], k = 28 Output: false

Constraints

• The number of nodes in the tree is in the range [1, 104].
• -104 <= Node.val <= 104
• root is guaranteed to be a valid binary search tree.
• -105 <= k <= 105

Thoughts

[!summary] This is a #BFS #hash_table problem.

Mainly two methods:

1. #BFS with hash table. Time space O(n) This can be quicker since you are starting at the middle, which is more likely to hit the answer, theoretically taking less time.
2. #binary_search. Time O(hn), h is the height of BST, best case h == log(n), worst case h == n for every node, binary search in the tree for the answer.

Solution

BFS with hash table

/**
 * 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:
  bool findTarget(TreeNode* root, int k) {
    // BFS with unordered_set
    // Take note: when to push root, when to push root->left and root->right
    unordered_set<int> uset;
    queue<TreeNode*> pending;
    pending.push(root);

    TreeNode* ptr;
    while(!pending.empty()) {
      ptr = pending.front();
      pending.pop();

      // find first, to avoid k = 10, val = 5
      if (uset.find(ptr->val) != uset.end()) {
        return true;
      }
      uset.insert(k - ptr->val);

      if (ptr->left) {
        pending.push(ptr->left);
      }
      if (ptr->right) {
        pending.push(ptr->right);
      }
    }

    return false;
  }
};