653. Two Sum IV - Input is a BST

Description

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

Problem URL

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Solution

在一棵BST中找two sum。

This is a sample approach which could be used in any tree. Using a set to store node value we have visited. Then DFS the whole tree to find if the set contains(k - node.val).

Code

/**
 * 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) {
        Set<Integer> set = new HashSet<>();
        return dfs(root, set, k);
    }
    
    private boolean dfs(TreeNode node, Set<Integer> set, int k){
        if (node == null){
            return false;
        }
        if (set.contains(k - node.val)){
            return true;
        }
        set.add(node.val);
        return dfs(node.left, set, k) || dfs(node.right, set, k);
    }
}

Time Complexity: O(n)
Space Complexity: O(n)

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Review

We could also store the values in BST into a array then perfom two sum using two pointers.