Partition List

Description

Given a linked list and a value x, partition it such that all nodes less than x come before nodes greater than or equal to x.

You should preserve the original relative order of the nodes in each of the two partitions.

Example:

Input: head = 1->4->3->2->5->2, x = 3
Output: 1->2->2->4->3->5

Solution

class Solution {
public:
    ListNode *partition(ListNode *head, int x) {
        ListNode dummyLeft(0), dummyRight(0);
        ListNode *left = &dummyLeft, *right = &dummyRight, *curr = head;
        while (curr) {
            if (curr->val < x) {
                left->next = curr;
                left = curr;
            } else {
                right->next = curr;
                right = curr;
            }
            curr = curr->next;
        }
        left->next = dummyRight.next;
        right->next = nullptr;
        return dummyLeft.next;
    }
};