[LeetCode] 450. Delete Node in a BST

 Problem : https://leetcode.com/problems/delete-node-in-a-bst/

/**
 * 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 root;
        }
        
        // delete node from left sub-tree
        if (root.val > key) {
            root.left = deleteNode(root.left, key);
            return root;
        }
        
        // delete node from right sub-tree
        if (root.val < key) {
            root.right = deleteNode(root.right, key);
            return root;
        }
        
        // root.val == key
        if (root.left == null) {
            return root.right;
        }
        
        if (root.right == null) {
            return root.left;
        }
        
        // use right node as the new root
        TreeNode newRoot = root.right;
        
        // Because all nodes in right sub-tree are larger than current root node,
        // and left child is less than current root node, 
        // then the left child must be the smallest value comparing to the right sub-tree.
        // Attach current left child to the most left node of the right sub-tree.
        TreeNode p = root.right;
        while (p.left != null) {
            p = p.left;
        }
        
        p.left = root.left;
        
        return newRoot;
    }
}