[LeetCode] 653. Two Sum IV - Input is a BST

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

Thought process:
In-order traversal.

Solution:

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/**
 * 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) {
        List<Integer> nodes = new ArrayList<>();
        inOrder(root, nodes);
        
        int i = 0;
        int j = nodes.size() - 1;
        while (i < j) {
            int left = nodes.get(i);
            int right = nodes.get(j);
            if (left + right < k) {
                i++;
            } else if (left + right == k) {
                return true;
            } else {
                j--;
            }
        }
        return false;
    }
    
    private void inOrder(TreeNode root, List<Integer> nodes) {
        if (root == null) {
            return;
        }
        
        inOrder(root.left, nodes);
        nodes.add(root.val);
        inOrder(root.right, nodes);
    }
}

Time complexity: O(n).
Location: Philadelphia, PA, USA

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