Leetcode 82. Remove duplicate elements in the sorted list II (iteration; recursion (not simple recursion))

Friday, March 26, 2021, the weather is fine [Don’t lament the past, don’t waste the present, don’t fear the future]

---

1. Introduction

82. Delete duplicate elements in the sorted list II

2. Solution

2.1 Iteration

The key point of the solution is: when you encounter duplicate elements and delete them, do not update pre, because they may cur->nextstill be duplicate nodes; only update when the cur->valsum is cur->next->valnot equal (to ensure that curit is not a duplicate node) pre.

class Solution {
    
    
public:
    ListNode* deleteDuplicates(ListNode* head) {
    
    
        ListNode* preHead = new ListNode(0);
        preHead->next = head;
        ListNode* pre = preHead, *cur = head;
        while(cur && cur->next){
    
    
            // 遇到重复元素，就彻底“消灭”它
            if(cur->val==cur->next->val) {
    
    
                int a = cur->val;
                while(cur->next && cur->next->val==a) cur = cur->next;
                pre->next = cur->next; // pre 不更新，因为可能 cur->next 还是重复的节点
            }
            else pre = pre->next; // 只有当 当前节点 和 下一节点 不相等时，才更新 pre
            cur = cur->next;
        }
        return preHead->next;
    }
};

2.2 Recursion

The recursion of this question is really not as simple as iterative. The specific ideas are as follows:

class Solution {
    
    
public:
    ListNode* deleteDuplicates(ListNode* head) {
    
    
        if(head==nullptr || head->next==nullptr) return head;
        if(head->val!=head->next->val){
    
    
            head->next = deleteDuplicates(head->next);
        }
        else{
    
    
            ListNode* move = head->next;
            while(move && move->val==head->val) move = move->next;
            return deleteDuplicates(move);
        }
        return head;
    }
};

---

references

https://leetcode-cn.com/problems/remove-duplicates-from-sorted-list-ii/solution/fu-xue-ming-zhu-di-gui-die-dai-yi-pian-t-wy0h/