Delete Node in a BST

/**
 * 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) {}
 * };
 */
class Solution {
public:

TreeNode* maxValue(TreeNode*root){
    if(root==NULL){
        return 0;
    }
    TreeNode*temp =root;
    while(temp->right!=NULL){
        temp= temp->right;
    }
    return temp;
}
    TreeNode* deleteNode(TreeNode* root, int key) {
        if(root==NULL)
            return root;

        if(root->val ==key){
            // for 0 child
            if(root->left==NULL && root->right ==NULL){
                delete root;
                return NULL;
            }
            // for 1 child
            if(root->left!=NULL && root->right ==NULL){
                TreeNode*temp = root->left;
                delete root;
                return temp;
            }
             if(root->left==NULL && root->right !=NULL){
                TreeNode*temp = root->right;
                delete root;
                return temp;
            }
            // for 2 child
            if(root->left!=NULL && root->right!=NULL){
            int maxi = maxValue(root->left)->val;
              root->val = maxi;
              root->left =deleteNode(root->left,maxi);
              return root;
            }
        }
        else if(root->val >key){
            root->left= deleteNode(root->left,key);
            return root;
        }
        else{
            root->right = deleteNode(root->right,key);
            return root;
        }
        return root;
    }
};