题意

给定一颗二叉搜索树，返回是否存在两个节点的值之和为给定值K.

思路

同Two Sum.使用Hash表解决。只是要写个树的遍历而已，选取DFS.

源码

class Solution {
    public boolean findTarget(TreeNode root, int k) {
        return work(root,k,new HashSet<Integer>());
    }
    private boolean work(TreeNode root,int k,Set<Integer>s) {
        if (root == null)
            return false;
        if (s.contains(k-root.val))
            return true;
        s.add(root.val);
        if (work(root.left,k,s) || work(root.right,k,s))
            return true;
        return false;
    }
}

结果
Runtime: 14 ms, faster than 85.59% of Java online submissions for Two Sum IV - Input is a BST.
Memory Usage: 43.9 MB, less than 100.00% of Java online submissions for Two Sum IV - Input is a BST.