0450-delete-node-in-a-bst.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public TreeNode deleteNode(TreeNode root, int key) {
        if (root == null) {
          return null;  
        } 
        if (root.val < key) {
            root.right = deleteNode(root.right, key);
        }
        else if (root.val > key) {
            root.left = deleteNode(root.left, key);
        }
        else {
            if (root.left == null) return root.right;
            if (root.right == null) return root.left;
            
            TreeNode minRight = root.right;
            while (minRight.left != null) {
                minRight = minRight.left;
            }
            minRight.left = root.left;
            return root.right;
        }
        return root;
    }
}