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

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */

// Time: O(h)
// Space: O(1)
class Solution {
public:
    TreeNode* deleteNode(TreeNode* root, int key) {
        if(root == nullptr) return root;

        if(root->val > key) {
            root->left = deleteNode(root->left, key);
        }
        else if(root->val < key) {
            root->right = deleteNode(root->right, key);
        }

        // perform deletion if root->val == key
        else {
            if(!root->left) return root->right;
            if(!root->right) return root->left;

            // both left and right subtrees are not null
            // traverse the right subtree to find the minimum/leftmost value
            // (or) travel the left subtree to find the maximum/rightmost value
            TreeNode* temp = root->right;
            while(temp->left) {
                temp = temp->left;
            }
            root->val = temp->val;

            root->right = deleteNode(root->right, temp->val);
        }

        return root;
    }
};