Interview Classic 150 Problem P86: Partition List

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Timeline

Timeline

2025-11-21

init

linked list

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Problem:

P86 Partition List

https://leetcode.cn/problems/partition-list/description/?envType=study-plan-v2&envId=top-interview-150

The meaning is to put those greater than k in front and those less than k in the back, but the relative order among the group greater than k remains unchanged, and the relative order among the group less than k also remains unchanged.

struct ListNode {
	int val;
	ListNode *next;
	ListNode()
		: val(0)
		, next(nullptr)
	{
	}
	ListNode(int x)
		: val(x)
		, next(nullptr)
	{
	}
	ListNode(int x, ListNode *next)
		: val(x)
		, next(next)
	{
	}
};

class Solution {
    public:
	ListNode *partition(ListNode *head, int x)
	{
		ListNode *virt_node = new ListNode;
		virt_node->next = head;

		ListNode *prev = virt_node, *p, *tmp;
		while (prev->next != nullptr && prev->next->val < x) {
			prev = prev->next; //Find the predecessor node of the first node greater than or equal to x.
		}

		if (prev->next == nullptr) { //all vals < x
			return head;
		}
		p = prev;

		while (p->next != nullptr) {
			if (p->next->val < x) {
				tmp = p->next;
				p->next = tmp->next;
				tmp->next = prev->next;
				prev->next = tmp;
				prev = tmp;
			} else {
				p = p->next;
			}
		}
		head = virt_node->next;
		delete virt_node;
		return head;
	}
};