Problem

Given a root node reference of a BST and a key, delete the node with the given key in the BST. Return the root node reference (possibly updated) of the BST.

Basically, the deletion can be divided into two stages:

Search for a node to remove.
If the node is found, delete the node.
Note: Time complexity should be O(height of tree).

Example:

root = [5,3,6,2,4,null,7]
key = 3

    5
   / \
  3   6
 / \   \
2   4   7

Given key to delete is 3. So we find the node with value 3 and delete it.

One valid answer is [5,4,6,2,null,null,7], shown in the following BST.

    5
   / \
  4   6
 /     \
2       7

Another valid answer is [5,2,6,null,4,null,7].

    5
   / \
  2   6
   \   \
    4   7

思路

1. compare The given node and tree node until the given node is found in the tree
2. four cases:
   1. node doesn't have left or right -
      1. return null
   2. node only has left subtree-
      1. return the left subtree
   3. node only has right subtree-
      1. return the right subtree
   4. node has both left and right -
      1. find the maximum value in the left subtree, connect right tree to the right of this left subtree
      2. return left

    public TreeNode removeNode(TreeNode root, int value) {
        if (root == null) return null;

        if (root.val > value) {
            root.left = removeNode(root.left, value);
        } else if (root.val < value) {
            root.right = removeNode(root.right, value);
        } else {
            if (root.left == null) {
                return root.right;
            } 
            if (root.right == null) {
                return root.left;
            }

            TreeNode temp = root.left;
            while (temp.right != null) {
                temp = temp.right;
            }
            temp.right = root.right;

            return root.left;
        }

        return root;
    }