Leetcode 445

Author: Kondareddy1209

Leetcode/Leetcode_445.py

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+# LeetCode 445 - Add Two Numbers II
+#
+# ---------------------------------------------
+# Approach 1: Convert to Integer (Brute Force)
+# ---------------------------------------------
+# Time Complexity : O(n + m)
+# Space Complexity: O(n + m)
+#
+# Idea:
+# 1. Store both linked lists into arrays.
+# 2. Convert arrays into integers.
+# 3. Add the integers.
+# 4. Convert the result back into a linked list.
+#
+# Note:
+# This solution is simple but does NOT satisfy the follow-up
+# because it converts the entire number into an integer.
+
+
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+
+
+class SolutionBruteForce:
+    def addTwoNumbers(self, l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:
+
+        n1 = []
+        curr = l1
+        while curr:
+            n1.append(curr.val)
+            curr = curr.next
+
+        n2 = []
+        curr = l2
+        while curr:
+            n2.append(curr.val)
+            curr = curr.next
+
+        num1 = int("".join(map(str, n1)))
+        num2 = int("".join(map(str, n2)))
+
+        total = num1 + num2
+
+        digits = list(map(int, str(total)))
+
+        dummy = ListNode(0)
+        curr = dummy
+
+        for digit in digits:
+            curr.next = ListNode(digit)
+            curr = curr.next
+
+        return dummy.next
+
+
+# ---------------------------------------------------
+# Approach 2: Using Two Stacks (Optimal)
+# ---------------------------------------------------
+# Time Complexity : O(n + m)
+# Space Complexity: O(n + m)
+#
+# Idea:
+# 1. Store digits of both linked lists into stacks.
+# 2. Pop digits from the end (least significant digit).
+# 3. Maintain carry.
+# 4. Insert each new node at the FRONT of the answer.
+#
+# This satisfies the follow-up (no integer conversion).
+
+
+class Solution:
+    def addTwoNumbers(self, l1: Optional[ListNode], l2: Optional[ListNode]) -> Optional[ListNode]:
+
+        stack1 = []
+        stack2 = []
+
+        curr = l1
+        while curr:
+            stack1.append(curr.val)
+            curr = curr.next
+
+        curr = l2
+        while curr:
+            stack2.append(curr.val)
+            curr = curr.next
+
+        carry = 0
+        dummy = ListNode(0)
+
+        while stack1 or stack2 or carry:
+
+            total = carry
+
+            if stack1:
+                total += stack1.pop()
+
+            if stack2:
+                total += stack2.pop()
+
+            carry = total // 10
+            digit = total % 10
+
+            # Insert at the beginning
+            node = ListNode(digit)
+            node.next = dummy.next
+            dummy.next = node
+
+        return dummy.next
+
+
+"""
+Example
+
+Input:
+l1 = 7 -> 2 -> 4 -> 3
+l2 = 5 -> 6 -> 4
+
+Approach 1:
+7243 + 564 = 7807
+Output:
+7 -> 8 -> 0 -> 7
+
+Approach 2:
+Stacks:
+stack1 = [7, 2, 4, 3]
+stack2 = [5, 6, 4]
+
+Pop:
+3 + 4 = 7
+4 + 6 = 10
+2 + 5 + 1 = 8
+7 + 0 = 7
+
+Build answer from front:
+
+7
+↓
+
+0 -> 7
+↓
+
+8 -> 0 -> 7
+↓
+
+7 -> 8 -> 0 -> 7
+
+Output:
+7 -> 8 -> 0 -> 7
+"""