DeleteNodeInBST.java

/**
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
 */
 
public class Solution {
    public TreeNode deleteNode(TreeNode root, int key) {
		if (root == null)
			return null;
		if (key < root.val) 
			root.left = deleteNode(root.left, key);
		else if (key > root.val) 
			root.right = deleteNode(root.right, key);
		else {
			if (root.left == null)
				return root.right;
			else if (root.right == null){
				return root.left;
			}
			else {
				TreeNode minNode = findMin(root.right);
				root.val = minNode.val;
				root.right = deleteNode(root.right, root.val);
			}
		}
		return root;
	}
	public TreeNode findMin(TreeNode node) {
		while (node.left != null) {
			node = node.left;
		}
		return node;
	}
}