Solving Skill

  class Solution {     public ListNode addTwoNumbers ( ListNode l1 , ListNode l2 ) {         ListNode head = new ListNode ( 0 );         ListNode curr = head;         int carry = 0 ;         while (l1 != null || l2 != null  || carry > 0 ) {             int val1 = l1 != null ? l1 . val : 0 ;             int val2 = l2 != null ? l2 . val : 0 ;             int sum = val1 + val2 + carry;             carry = sum / 10 ;             curr . next = new ListNode (sum % 10 );             curr = curr . next ;             l1 = l1 != null ? l1 . next : null ;             l2 = l2 != null ? l2 . next : null ;      ...

  Solution 1: Brute Force class Solution {      public int [] twoSum ( int [] nums , int target ) {          int len = nums . length ;          for ( int i = 0 ; i < len; i++) {              for ( int j = i + 1 ; j < len; j++) {                  if (nums[i] + nums[j] == target)                      return new int [] {i, j};             }         }          return null ;     } } Complexity Time: O(n^2) Complexity Space: O(1) Solution 2: Using Hashmap class Solution { ...