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

题意：给定一棵二叉查找树和一个目标值target，判断BST中是否存在两个元素使其相加和为target
思路：用深度优先搜索，将二叉树中的所有节点值放入ArrayList，然后判断是否存在相加为target的两个元素，实现如下：

public boolean findTarget(TreeNode root, int k) {
        List<Integer> nodes = new ArrayList<>();
        dfs(root, nodes);
        for(int i = 0; i < nodes.size(); i++){
            int val = nodes.get(i);
            if(nodes.contains(k - val) && k - val != val)
                return true;
        }
        return false;
    }
    public void dfs(TreeNode root, List<Integer> nodes){
        if(root != null){
            nodes.add(root.val);
            dfs(root.left, nodes);
            dfs(root.right, nodes);
        }
    }