21.Merge Two Sorted Lists (List)

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        if(l1 == NULL) return l2;
        if(l2 == NULL) return l1;
        
        ListNode* root = l1;
        if(l1->val <= l2->val){
            root = l1;
            l1 = l1->next;
        }
        else{
            root = l2;
            l2 = l2->next;           
        }
        
        ListNode* current = root;
        while(l1 && l2){
            if(l1->val <= l2->val){
                current->next = l1;
                l1 = l1->next;
            }
            else{
                current->next = l2;
                l2 = l2->next;           
            }
            current = current->next;
        }
        
        if(l1){
            current->next = l1;
        }
        else if(l2){
            current->next = l2;
        }
        
        return root;
    }
};