653. Two Sum IV - Input is a BST
Tags: ‘Tree’
Given a Binary Search Tree and a target number, return true if there exist two elements in the BST such that their sum is equal to the given target.
Example 1:
Input:
5
/ \
3 6
/ \ \
2 4 7
Target = 9
Output: True
Example 2:
Input:
5
/ \
3 6
/ \ \
2 4 7
Target = 28
Output: False
Solution
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public boolean findTarget(TreeNode root, int k) {
//Recursive, using hashset
Set<Integer> set = new HashSet<>();
return find(root, k, set);
}
private boolean find(TreeNode root, int k, Set<Integer> set) {
// preorder traversal
if (root == null) return false; // end case
if (set.contains(k - root.val)) return true;
set.add(root.val);
return find(root.left, k, set) || find(root.right, k, set);
}
}
class Solution {
public boolean findTarget(TreeNode root, int k) {
// iterative bfs, using hashset
Set<Integer> set = new HashSet<>();
Queue<TreeNode> queue = new LinkedList<>();
queue.add(root);
while (!queue.isEmpty()) {
if (queue.peek() != null) {
TreeNode node = queue.remove();
if (set.contains(k - node.val)) return true;
set.add(node.val);
queue.add(node.left);
queue.add(node.right);
} else {
queue.remove();
}
}
return false;
}
}