输入值是指针考虑是不是NULL， next是不是NULL
输入值是数组考虑长度为0

Search a 2D Matrix II

Instead of beginning with matrix[0][0], begin with left bottom or right up - > then when greater or less, the direction is only.

Swap Nide in Pairs

ListNode* swapPairs(ListNode* head) {
    if(head==NULL) return NULL;
    if(head->next==NULL) return head;
    ListNode* newhead = head->next;
    while(head!=NULL&&head->next!=NULL){
        ListNode* second = head->next;
        ListNode* first = head;
        head = second->next;
        if(second->next==NULL||second->next->next==NULL) first->next = second->next;
        else first->next = second->next->next;
        second->next = first;
    }
    return newhead;
}

Pay attenion to where first->next should be point to, not simple second->next or second->next->next as well as the condition in while.

    
    ListNode* swapPairs(ListNode* head) {
        if(head==NULL) return NULL;
        if(head->next==NULL) return head;
        ListNode* newhead = head->next;
        head->next = swapPairs(head->next->next);
        newhead->next = head;
        return newhead;
    }

Container With Most Water

int maxArea(vector<int>& height) {
    int left = 0;
    int right = height.size()-1;
    int max = 0;
    while(left<right){
        if(height[left]<height[right]){
            int area = height[left]*(right - left);
            if(max<area) max = area;
            left++;
        }
        else{
            int area = height[right]*(right - left);
            if(max<area) max = area;
            right--;
            
        }
    }
    return max;
}

Use two pointer is fine, but need to consider the circumstance that even current boundary has greater area then adjancent one, there may be much higher height in middle, so still need to search to center.