#40 Medium Time O(n) Space O(n) Animated

Add Two Numbers

Add two numbers represented as linked lists.

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Tutor Mode Build the mental model before you memorize the code.

1 Predict
2 Trace
3 Explain
4 Implement

Pattern Pointer rewiring with saved next state

Before changing a link, save the only pointer that would otherwise be lost.

Approach: Math with Carry

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Core Insight

Before changing a link, save the only pointer that would otherwise be lost.

Common Pitfall

Always store next before redirecting current.next.

Description

Add two numbers represented as linked lists.

Constraints

Time Complexity: O(n)
Space Complexity: O(n)

Solution — Math with Carry

TimeO(n) SpaceO(n)

Implementation Plan

Translate the invariant into these small, testable moves.

Initialize the pointers.
Save the next node.
Rewire the current node.
Advance both pointers until the list is consumed.

def addTwoNumbers(l1, l2):
    dummy = ListNode()
    curr = dummy
    carry = 0
    
    while l1 or l2 or carry:
        v1 = l1.val if l1 else 0
        v2 = l2.val if l2 else 0
        
        val = v1 + v2 + carry
        carry = val // 10
        val = val % 10
        curr.next = ListNode(val)
        
        curr = curr.next
        l1 = l1.next if l1 else None
        l2 = l2.next if l2 else None
        
    return dummy.next

Copy List with Random Pointer

Linked List Cycle

Add Two Numbers

Steps

Description

Constraints

Tags

Solution — Math with Carry