Delete_Node_in_a_BST.java

/*
450. Delete Node in a BST
--------------------------

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.
Follow up: Can you solve it with time complexity O(height of tree)?

Example 1:
Input: root = [5,3,6,2,4,null,7], key = 3
Output: [5,4,6,2,null,null,7]
Explanation: 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 above BST.
Please notice that another valid answer is [5,2,6,null,4,null,7] and it's also accepted.

Example 2:
Input: root = [5,3,6,2,4,null,7], key = 0
Output: [5,3,6,2,4,null,7]
Explanation: The tree does not contain a node with value = 0.

Example 3:
Input: root = [], key = 0
Output: []

*/

class Solution {
    
    public int Successor(TreeNode root){
        root=root.right;
        while(root.left!=null)
            root=root.left;
        return root.val;
    }
    
    public int Predecessor(TreeNode root){
        root=root.left;
        while(root.right!=null)
            root=root.right;
        return root.val;
    }
    
    public TreeNode deleteNode(TreeNode root, int key) {
        if(root==null)
            return root;
        if(key>root.val)
            root.right=deleteNode(root.right,key);
        else if(key<root.val)
            root.left=deleteNode(root.left,key);
        else{
            if(root.left==null && root.right==null)
                root=null;
            else if(root.right!=null){
                root.val=Successor(root);
                root.right=deleteNode(root.right,root.val);
            }
            else{
                root.val=Predecessor(root);
                root.left=deleteNode(root.left,root.val);
            }
        }
        return root;
    }
}