Abstract / Summary

Given an integer array and a target value, calculate the total value by concatenating each element with the target and converting to integers.

Note

The full problem description, including helpful examples, can be found on LeetCode.

This problem is ranked as

and has an acceptance rate of about 72% (90k+ submissions).

Typical patterns to solve this problem:

• Array
• Simulation
• Two Pointers

Every single line of code was written by me, 100% from scratch, between August 2023 and August 2024.
However, two years later I set up this website and used various AI tools to generate most of the walkthrough / explanations below.

See this dedicated page for more details.

Overview

The provided implementation defines a single method findTheArrayConcVal that computes a numeric value derived from a list of integers. The algorithm repeatedly pairs the first and last elements of the list, concatenates their decimal representations, converts the concatenated string back to an integer, and accumulates the sum. When fewer than two elements remain, any leftover element is added directly to the total. The method returns the final accumulated sum.

Key Steps

The computation proceeds through a clear sequence of operations, each reflected in a concise code fragment.

Initialization

result = 0

A running total named result is created and set to zero. This variable will hold the cumulative sum of all concatenated values produced during the loop.

Loop Condition

while len(nums) >= 2:

The loop continues as long as the list contains at least two elements. This condition guarantees that a pair can be formed from the front and back of the list on each iteration.

Forming the Concatenated Number

concat = str(nums[0]) + str(nums[-1])

The first element (nums[0]) and the last element (nums[-1]) are each converted to their string representation. The two strings are then concatenated, producing a new string that represents the decimal concatenation of the two numbers. For example, if the pair is 12 and 34, the resulting string is "1234".

Updating the Accumulator

result += int(concat)

The concatenated string is converted back to an integer, and this integer is added to the running total result. This step integrates the contribution of the current pair into the final answer.

Removing Processed Elements

nums.pop(0)
nums.pop()

The first element is removed with pop(0), and the last element is removed with pop() (which defaults to the final position). By mutating the original list in place, the algorithm avoids allocating additional data structures to track which elements have been processed.

Handling the Remaining Element

return result + nums[0] if nums else result

When the loop exits, the list may contain a single leftover element. The conditional expression checks whether any element remains. If so, that element is added directly to result; otherwise, result is returned unchanged. This final step ensures that an odd-length input list contributes all of its numbers to the total.

Edge Cases Handled

The implementation gracefully manages several boundary conditions without requiring explicit checks elsewhere in the code.

When the input list is empty, the loop body is never entered because len(nums) >= 2 evaluates to false. The conditional return expression evaluates the else branch, yielding result, which remains zero. Thus the function returns 0 for an empty list.

When the input list contains exactly one element, the loop condition fails immediately, and the conditional return expression adds that sole element to result. Consequently the function returns the value of the single element, matching the expected behavior for a list that cannot be paired.

When the list contains two elements, the loop executes once. The concatenation step combines the two numbers, the sum is updated, and both elements are removed. After the loop, the list is empty, so the return expression yields the accumulated result. This correctly reflects the single concatenated pair.

The conversion to strings and back to integers automatically handles numbers with varying digit lengths, including those that start with zero when represented as strings (e.g., 0 and 5 become "05" which converts to 5). The integer conversion discards any leading zeros, preserving the numeric meaning of the concatenated value.

Performance Optimizations Observed

The algorithm operates directly on the input list, modifying it in place rather than constructing auxiliary collections. By using pop() for the last element and pop(0) for the first, the code eliminates the need for index arithmetic or slicing, which could introduce additional overhead. The use of a simple while loop with a clear termination condition keeps the control flow straightforward and minimizes branching.

String concatenation is performed using the + operator on two short strings, which is efficient for the small number of characters involved in each iteration. Converting the concatenated string to an integer only once per pair avoids repeated parsing.

Overall, the method balances readability with in-place data manipulation, achieving the desired result with a minimal set of operations.

Imagine a magic train that carries a line of numbered boxes. This code acts like a helper who wants to combine these numbers into a big number. The helper always looks at the very first box in the line and the very last box in the line.

To combine them, the helper writes the numbers down on a piece of paper side by side. For example, if the first box has a one and the last box has a five, they turn into fifteen. Once those two numbers are taped together to make a new, bigger number, they are taken off the train so they do not get in the way.

The helper keeps doing this over and over again, grabbing the new first and last boxes, taping them together, and adding that new value to a growing pile. If there is only one box left at the end because the line had an odd number of boxes, the helper just takes that last number and adds it to the pile to finish the job.

Several LLMs ranked my code quality on a scale from 1 (very poor) to 10 (outstanding):

not analyzed yet

I wrote a basic crawler to find other solutions to that problem.
Obviously, I did that after I solved the problem myself.

In general, AlgoMonster and WalkCCC solutions are high-quality and always worth a look.
I haven't checked most of the other solutions, but they might be helpful as well.

And of course, the LeetCode forum is always a good place (or even the best) to look for alternative approaches.

(click on a column header to sort the table)

	author
author	solutions
www	AlgoMonster	Java	Python		C++				TS
	WalkCCC				C++
	leetcode.ca	Java	Python		C++	C	Go	Rust	TS
	Progiez	Java	Python		C++
github	charles-wangkai	Java
	fsq				C++
	JoshCrozier			JS
	lkwq007		Python
	varunu28	Java
AlgoMonster	C++, Python, Java, TypeScript
WalkCCC	C++
leetcode.ca	C, C++, Go, Java, Python, Rust, TypeScript
Progiez	C++, Java, Python
charles-wangkai	Java
fsq	C++
JoshCrozier	JavaScript
lkwq007	Python
varunu28	Java

There are a few extra pages giving you some insights into my setup:

• Tech Stack - my development environment and tools
• AI - how AI helped me create this website (NOT solving the problems)
• Progress - progression over time
• Quality - code quality