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AC代码：

/**
 * 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) {
        if(root==null){
            return node;
        }else if(root.val<=node.val){
            root.right=insertNode(root.right,node);
        }else{
            root.left=insertNode(root.left,node);
        }
        return root;
    }
}

题目来源： http://www.lintcode.com/zh-cn/problem/insert-node-in-a-binary-search-tree/