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

S: recursion

根据BST的特性，删除一个节点时会由该节点右子树的最左节点代替该节点的位置

    TreeNode* deleteNode(TreeNode* root, int key) {
        if (!root) return NULL;
        if (root->val == key) {
            if (root->right && root->left) {
                TreeNode* node = root->right;
                while (node->left) {
                    node = node->left;
                }
                node->left = root->left;
                return root->right;
            } else {
                return root->right? root->right: root->left;
            }
        } else if (root->val < key) {
            root->right = deleteNode(root->right, key);
        } else {
            root->left = deleteNode(root->left, key);
        }
        return root;   
    }