删除二叉搜索树中的节点

class Solution {
public:
    TreeNode* pre=NULL;
    TreeNode* deleteNode(TreeNode* root, int key){
        if(root==NULL) return NULL;
        if(root->val==key){
            if(root->left==NULL&&root->right==NULL){
                if(pre==NULL) return NULL;
                if(pre!=NULL&&pre->val<key) pre->right=NULL;
                if(pre!=NULL&&pre->val>key) pre->left=NULL;
                return root;
            }
            if(root->left==NULL){
                if(pre==NULL) root=root->right;
                if(pre!=NULL&&pre->val<key) pre->right=root->right;
                if(pre!=NULL&&pre->val>key) pre->left=root->right;
                return root;
            }
            if(root->right==NULL){
                if(pre==NULL) root=root->left;
                if(pre!=NULL&&pre->val<key) pre->right=root->left;
                if(pre!=NULL&&pre->val>key) pre->left=root->left;
                return root;
            }
            if(pre!=NULL&&pre->val<key) pre->right=root->left;
            if(pre!=NULL&&pre->val>key) pre->left=root->left;
            pre=root->left->right;
            root->left->right=root->right;
            TreeNode* cur=root->right;
            while(cur->left!=NULL){
                cur=cur->left;
            }
            cur->left=pre;
            return root->left;
        }
        pre=root;
        if(root->val>key) deleteNode(root->left, key);
        if(root->val<key) deleteNode(root->right, key);
        return root;
    }
};