Sum Lists

Author: anantkaushik

linked-lists/sum-lists.py

@@ -0,0 +1,70 @@
+"""
+You have two numbers represented by a linked list, where each node contains a single digit. The digits are stored in reverse
+order, such that the 1's digit is at the head of the list. Write a function that adds the two numbers and returns the sum as
+the linked list. 
+
+Example:
+Input: (7 -> 1 -> 6) + (5 -> 9 -> 2). That is, 617 + 295.
+Output: 2 -> 1 -> 9. That is, 912.
+
+FOLLOW UP:
+Suppose the digits are stored in forward order. Repeat the above problem.
+Input: (6 -> 1 -> 7) + (2 -> 9 -> 5). That is, 617 + 295.
+Output: 9 -> 1 -> 2. That is, 912.
+"""
+def addTwoNumbers(l1, l2):
+    res = ListNode(0)
+    cur = res
+    carry = 0
+    while l1 or l2:
+        x = l1.val if l1 else 0
+        y = l2.val if l2 else 0
+        summ = carry + x + y
+        carry = summ // 10
+        cur.next = ListNode(summ%10)
+        cur = cur.next
+        if l1:
+            l1 = l1.next
+        if l2:
+            l2 = l2.next
+    if carry > 0:
+        cur.next = ListNode(carry)
+    return res.next
+
+""" Follow Up """
+class Solution(object):
+    def addTwoList(self, l1, l2):
+        len1,len2 = self.getLength(l1),self.getLength(l2)
+        if len1 > len2:
+            l2 = self.addLeadingZeroes(len1-len2,l2)
+        else:
+            l1 = self.addLeadingZeroes(len2-len1,l1)
+        carry, summ = self.addNumbers(l1,l2)
+        if carry > 0:
+            new = ListNode(carry)
+            new.next = summ
+            summ = new
+        return summ
+        
+    def getLength(self, node):
+        count = 0
+        while node:
+            count += 1
+            node = node.next
+        return count
+    
+    def addLeadingZeroes(self,count,node):
+        for i in range(count):
+            new = ListNode(0)
+            new.next = node
+            node = new
+        return node
+    
+    def addNumbers(self,l1,l2):
+        if not l1 and not l2:
+            return (0,None)
+        carry,new = self.addNumbers(l1.next,l2.next)
+        summ = l1.val + l2.val + carry
+        ans = ListNode(summ%10)
+        ans.next = new
+        return (summ//10,ans)