add code of 2

Author: Li Li

top-interview-questions/2. Add Two Numbers.cs

@@ -0,0 +1,64 @@
+// O(1) space
+public class Solution {
+    public ListNode AddTwoNumbers(ListNode l1, ListNode l2) {
+        if (l1 == null) return l2;
+        if (l2 == null) return l1;
+        ListNode dummy = new ListNode(0);
+        ListNode head = dummy;
+        int carry = 0;
+        int sum = 0;
+        while (l1 != null && l2 != null) {
+            head.next = l1;
+            head = head.next;
+            sum = l1.val + l2.val + carry;
+            head.val = sum % 10;
+            carry = sum / 10;
+            l1 = l1.next;
+            l2 = l2.next;
+        }
+        if (l1 != null) {
+            head.next = l1;
+        } 
+        if (l2 != null) {
+            head.next = l2;
+        }
+        
+        while (head.next != null) {
+            head = head.next;
+            sum = head.val + carry;
+            head.val = sum % 10;
+            carry = sum / 10;
+            if (carry == 0) break;
+        }
+        if (carry != 0) {
+            head.next = new ListNode(carry);
+        }
+        return dummy.next;
+    }
+}
+
+// O(m+n) space
+public class Solution {
+    public ListNode AddTwoNumbers(ListNode l1, ListNode l2) {
+        if (l1 == null) return l2;
+        if (l2 == null) return l1;
+        ListNode dummy = new ListNode(0) { next = l1 };
+        ListNode cur = dummy;
+        int carry = 0;
+        int sum = 0;
+        while (l1 != null || l2 != null) {
+            int x = (l1 != null) ? l1.val : 0;
+            int y = (l2 != null) ? l2.val : 0;
+            sum = x + y + carry;
+            cur.next = new ListNode(sum % 10);
+            carry = sum / 10;
+            if (l1 != null) l1 = l1.next;
+            if (l2 != null) l2 = l2.next;
+            cur = cur.next;
+        }
+        if (carry != 0) {
+            cur.next = new ListNode(carry);
+        }
+        return dummy.next;
+    }
+}