問題： Given the root node of a binary search tree (BST) and a value to be inserted into the tree, insert the value into the BST. Return the root node of the BST after the insertion. It is guaranteed that the new value does not exist in the original BST. Note that there may exist multiple valid ways for the insertion, as long as the tree remains a BST after insertion. You can return any of them.

For example, 

Given the tree:
        4
       / \
      2   7
     / \
    1   3
And the value to insert: 5
You can return this binary search tree:

         4
       /   \
      2     7
     / \   /
    1   3 5
This tree is also valid:

         5
       /   \
      2     7
     / \   
    1   3
         \
          4
 

方法： 二叉搜尋樹的遞迴，重點在於理順遞迴呼叫邏輯，具體實現參考程式碼。

具體實現：

class InsertIntoABinarySearchTree {

    // Definition for a binary tree node.
    class TreeNode(var `val`: Int = 0) {
        var left: TreeNode? = null
        var right: TreeNode? = null
    }

    fun insertIntoBST(root: TreeNode?, `val`: Int): TreeNode? {
        if (root == null) {
            return root
        }
        if (`val` < root.`val`) {
            if (root.left == null) {
                root.left = TreeNode(`val`)
            } else {
                insertIntoBST(root.left, `val`)
            }
        } else {
            if (root.right == null) {
                root.right = TreeNode(`val`)
            } else {
                insertIntoBST(root.right, `val`)
            }
        }
        return root
    }
}

fun main(args: Array<String>) {

}
 

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