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難度：容易

要求：

給定一棵二叉查找樹和一個新的樹節點，將節點插入到樹中。
你需要保證該樹仍然是一棵二叉查找樹。

  2             2
 / \           / \
1   4   -->   1   4
   /             / \ 
  3             3   6

思路：遞歸

/**
 * 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 root: The root of the binary search tree.
     * @param node: insert this node into the binary search tree
     * @return: The root of the new binary search tree.
     */
    public TreeNode insertNode(TreeNode root, TreeNode node) {
        // write your code here
        if(root == null || node == null){
            return node;
        }
        int result = node.val - root.val;
        if(result < 0){
            root.left = insertNode(root.left, node);
        }else if(result > 0){
            root.right = insertNode(root.right, node);
        }
        return root;
    }
}