Two Sum IV - Input is a BST

Problem

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

Input: 
    5
   / \
  3   6
 / \   \
2   4   7

Target = 9

Output: True
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Input: 
    5
   / \
  3   6
 / \   \
2   4   7

Target = 28

Output: False

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Thinking

先将BST中的value存放到list中，再用双指针

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Answer

public class Solution {

	public boolean findTarget(TreeNode root, int k) {
		List<Integer> collector = new ArrayList<>();
		save(collector, root);
		return isTwoSum(collector, k);
	}

	// 将BST的值存到List中
	public void save(List<Integer> collector, TreeNode x){
		if (x == null)
			return;
		save(collector, x.left);
		collector.add(x.val);
		save(collector, x.right);
	}

	public boolean isTwoSum(List<Integer> collector, int k){
		int low = 0, high = collector.size()-1;
		while (low < high){
			int t = collector.get(low) + (int)collector.get(high);
			if (t < k) low++;
			else if (t > k) high--;
			else return true;
		}
		return false;
	}

}