# Delete Node in a BST

Nov 28, 2019

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

5
/ \
2   6
\   \
4   7


• leftright都为空：直接删除，返回nullptr即可
• leftright都不为空：默认采用把右子树的节点拉上来的方式，即把左子树插入到右子树中，然后再返回right即可。
• leftright有一个不为空，则返回不为空的子树即可。

    TreeNode* deleteNode(TreeNode *node) {
auto left = node->left, right = node->right;
delete node;
if (left && right) {
auto temp = right;
while(temp->left) {
temp = temp->left;
}
temp->left = left;
return right;
}
return (left ? left : (right ? right : nullptr));
}
TreeNode* deleteNode(TreeNode* root, int key) {
if (root == nullptr) return nullptr;
else if (root->val == key) {
return deleteNode(root);
} else if (key >  root->val) {
root->right = deleteNode(root->right, key);
} else
root->left = deleteNode(root->left, key);

return root;
}

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