450

/**
 * 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* deleteNode(TreeNode* root, int key) {
        if(!root) return NULL;
        if(key > root->val){
            root->right = deleteNode(root->right, key);
        }
        else if(key < root->val){
            root->left = deleteNode(root->left, key);
        }
        else{
            if(!root->left && !root->right){
                root = NULL;
            }
            else if(root->right){
                root->val = successor(root);
                root->right = deleteNode(root->right, root->val);
            }
            else{
                root->val = predecessor(root);
                root->left = deleteNode(root->left, root->val);
            }
        }
        return root;
    }

    int successor(TreeNode *root){
        root = root->right;
        while(root->left){
            root=root->left;
        }
        return root->val;
    }

    int predecessor(TreeNode *root){
        root = root->left;
        while(root->right){
            root = root->right;
        }
        return root->val;
    }

};