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.

Solution1 :

class Solution {

    public boolean findTarget(TreeNode root, int k) {

         HashSet<Integer> s = new HashSet<>(); //使用HashTable，检验是否存在k-root.val

         return dfs(root, s, k);        

    }

 

    private boolean dfs(TreeNode root, HashSet<Integer> s, int k) {

         if (root == null)

             return false;

         if (s.contains(k - root.val))

             return true;

         s.add(root.val);

         return dfs(root.left, s, k) || dfs(root.right, s, k);

    }

}

 

Solution2://使用ArrayList存一个有序的数组，二叉树里的值从小到大排列（inorder方法的作用）

class Solution {

    public boolean findTarget(TreeNode root, int k) {

        List<Integer> l = new ArrayList<>();

        inorder(root, l);

        for (int i = 0, j = l.size() - 1; i < j;) {

            if (l.get(i) + l.get(j) == k)

                return true;

            else if (l.get(i) + l.get(j) < k)

                i++;

            else j--;

        }

        return false;

    }

 

    private void inorder(TreeNode root, List<Integer> l) {

        if (root == null)

            return ;

        inorder(root.left, l);

        l.add(root.val);

        inorder(root.right, l);

    }

}