Two Sum

Brute Force: time O(n^2) space O(1)

Hash Map: time O(n) space O(n)

Add Two Numbers

Unconsidered:

[1]+[9,9]-> after l1 ends and l2 ends, there is still one carry

Struct initiation in C++: new ListNode(0)

if (temp->val==0) temp==0 

change to 

if (temp->val==0) {

l4 = l3;

while(l4->next!=NULL){

if(l4 ->next==temp) l4->next = NULL;

else l4=l4->next;

}

}

first one simply let temp points to NULL, but doesn't change the front node's next pointer, so l4 is needed as the front node then set its next to NULL.