[Leetcode]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:

1. Search for a node to remove.
2. 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


思路：分两种情况，一种是要删除的节点只有一个孩子，另一种是要删除的节点有两个孩子；
如果只有一个孩子，那么我们让这个节点引用到他非空的孩子上去,即root = root.right或
root = root.left;如果有两个孩子的话，那么有两种办法，一种是找他右边的最小值，另一种
是找他左边的的最大值。将值赋给该节点，然后删除右边最小值或左边最大值；

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode deleteNode(TreeNode root, int key) {
        if (root==null)
            return null;
        else 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&&root.right!=null){
                TreeNode tmp = findMin(root.right);
                root.val = tmp.val;
                root.right = deleteNode(root.right,root.val);
            }
            else {
                TreeNode tmp = root;
                if (root.left==null)
                    root = root.right;
                else if (root.right==null)
                    root = root.left;
                tmp = null;
            }
        }
        return root;
    }
    private TreeNode findMin(TreeNode root){
        if (root==null)
            return null;
        if (root.left==null)
            return root;
        return findMin(root.left);
    }
}