86 Partition List

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.
For example,

Given 1->4->3->2->5->2 and x = 3,
return 1->2->2->4->3->5.

Solution

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* partition(ListNode* head, int x) {
        ListNode *s = new ListNode(0), *g = new ListNode(0);
        ListNode *ps = s, *pg = g;
        ListNode *p = head;
        while ( p != NULL ) {
            if ( p->val < x ) { ps->next = p; ps = ps->next; }
            else { pg->next = p; pg = pg->next; }
            p = p->next;
        }
        ps->next = g->next;
        pg->next = NULL;
        return s->next;
    }
};