给一棵二叉搜索树以及一个整数 n, 在树中找到和为 n 的两个数字

样例

给一棵BST:

    4
   / \
  2   5
 / \
1   3

以及一个整数 n = 3
返回 [1, 2] 或 [2, 1]

注意事项

Without any extra space.

解题思路1：

先中序遍历将结果保存在HashSet中，然后再寻找。

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */

public class Solution {
    /*
     * @param : the root of tree
     * @param : the target sum
     * @return: two numbers from tree which sum is n
     */
    public int[] twoSum(TreeNode root, int n) {
        // write your code here
        Set<Integer> set = new HashSet<>();
        int[] res = new int[2];
        
        if(root == null)
            return null;
            
        inOrder(root, set);
        
        for(Integer i : set){
            int temp = n - i;
            
            if(set.contains(temp)){
                res[0] = i;
                res[1] = temp;
                
                return res;
            }
        }
        
        return null;
    }
    
    private void inOrder(TreeNode root, Set<Integer> set){
        if(root == null)
            return;
            
        inOrder(root.left, set);
        set.add(root.val);
        inOrder(root.right, set);
    }
}