LeetCode/cs/delete-node-in-a-bst.cs

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int val=0, TreeNode left=null, TreeNode right=null) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
public class Solution {
    public TreeNode DeleteNode(TreeNode root, int key) {
        if (root == null) {
            return null;
        }

        if (key == root.val) {
            if (root.left == null) {
                return root.right;
            }

            if (root.right == null) {
                return root.left;
            }

            // find successor
            var node = root.right;
            while (node.left != null) {
                node = node.left;
            }

            root.val = node.val;
            root.right = DeleteNode(root.right, node.val);
        } else if (key < root.val) {
            root.left = DeleteNode(root.left, key);
        } else if (key > root.val) {
            root.right = DeleteNode(root.right, key);
        }

        return root;
    }
}