LeetCode 2695 - Array Wrapper

This problem asks us to implement a class ArrayWrapper that encapsulates an array of integers and supports two operations in a custom way.

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LeetCode Problem 2695

Difficulty: 🟢 Easy
Topics: —

Solution

Problem Understanding

This problem asks us to implement a class ArrayWrapper that encapsulates an array of integers and supports two operations in a custom way. First, when two instances of ArrayWrapper are added together using the + operator, the result should be the sum of all elements in both arrays. This is a non-standard behavior because normally adding objects would not automatically sum their internal arrays, so we need operator overloading. Second, the class must support string conversion so that calling String() (or str() in Python) on an instance returns a string representing the array in standard bracket notation, with elements separated by commas.

The input represents an array of integers passed to the constructor. The output depends on the operation: either an integer sum when using the + operator, or a formatted string when using String().

Constraints tell us that the array can be empty, can contain up to 1000 elements, and each element is between 0 and 1000. This guarantees that sums will not be excessively large, so integer overflow is not a concern in Python, but in Go, we may need to use int carefully. Important edge cases include empty arrays, arrays of length 1, and arrays with the maximum allowed values.

Approaches

The brute-force approach is straightforward: store the array in the class and, when two instances are added, iterate through both arrays and sum the elements. For string conversion, manually concatenate the elements with commas inside brackets. This works correctly but requires explicit loops, which is trivial for small arrays but verbose.

The key insight for the optimal solution is that Python (and Go) provide built-in mechanisms for operator overloading and joining arrays into strings. In Python, we can define __add__ and __str__ methods, which automatically handle the custom behaviors. In Go, we use a method on a struct and define a String() method to implement the string formatting. The optimization is not in time complexity but in code simplicity and readability, leveraging built-in functions.

Approach Time Complexity Space Complexity Notes
Brute Force O(n + m) O(1) Explicitly iterate through arrays on addition and manually construct strings
Optimal O(n + m) O(1) Uses built-in sum and join methods for clarity and conciseness

Algorithm Walkthrough

  1. Define a class ArrayWrapper that stores the input array as an instance variable. This ensures all operations can access the internal array.
  2. Implement the addition operation. In Python, this is done with the __add__ method. Take another instance of ArrayWrapper and compute the sum of the two arrays by using the built-in sum() function. Return the resulting integer.
  3. Implement string conversion. In Python, define the __str__ method to convert the internal array to a comma-separated string enclosed in brackets. Use str.join() to concatenate elements efficiently.
  4. In Go, define a struct with a slice of integers, implement a method Add that takes another struct and returns the sum of slices, and implement the String() method to produce the bracketed comma-separated format.
  5. Ensure edge cases like empty arrays are handled naturally: summing empty arrays returns 0, and joining empty arrays returns "[]".

Why it works: By storing the array internally and leveraging built-in functions for summing and string conversion, the class guarantees that operations respect the intended semantics. Operator overloading ensures the + operator behaves as specified.

Python Solution

from typing import List

class ArrayWrapper:
    def __init__(self, nums: List[int]):
        self.nums = nums

    def __add__(self, other: "ArrayWrapper") -> int:
        return sum(self.nums) + sum(other.nums)

    def __str__(self) -> str:
        return "[" + ",".join(str(num) for num in self.nums) + "]"

The __init__ method stores the array in the instance. The __add__ method receives another ArrayWrapper and sums both internal arrays, returning the integer result. The __str__ method converts the internal array to the required bracketed string format using list comprehension and join.

Go Solution

package main

import (
    "fmt"
    "strings"
    "strconv"
)

type ArrayWrapper struct {
    nums []int
}

func Constructor(nums []int) ArrayWrapper {
    return ArrayWrapper{nums}
}

func (a ArrayWrapper) Add(other ArrayWrapper) int {
    sum := 0
    for _, num := range a.nums {
        sum += num
    }
    for _, num := range other.nums {
        sum += num
    }
    return sum
}

func (a ArrayWrapper) String() string {
    strNums := make([]string, len(a.nums))
    for i, num := range a.nums {
        strNums[i] = strconv.Itoa(num)
    }
    return "[" + strings.Join(strNums, ",") + "]"
}

In Go, the struct ArrayWrapper holds a slice of integers. The Add method explicitly loops over both slices to calculate the sum. The String method converts each integer to a string and joins them with commas inside brackets. Go requires more manual handling because slices and strings are not as flexible as Python lists and join.

Worked Examples

Example 1: nums = [[1,2],[3,4]], operation = "Add"

Step obj1.nums obj2.nums sum
Initialize [1,2] [3,4] -
Add - - 1+2 + 3+4 = 10

Example 2: nums = [[23,98,42,70]], operation = "String"

Step obj.nums str(obj)
Initialize [23,98,42,70] -
String - "[23,98,42,70]"

Example 3: nums = [[],[]], operation = "Add"

Step obj1.nums obj2.nums sum
Initialize [] [] -
Add - - 0+0 = 0

Complexity Analysis

Measure Complexity Explanation
Time O(n + m) Summing two arrays requires iterating through both arrays of length n and m
Space O(1) No additional data structures are used beyond storing the array

The complexity is linear in the size of the arrays being added or converted to a string. Since the arrays are stored internally, no extra space beyond the input is required.

Test Cases

# Provided examples
assert ArrayWrapper([1,2]).__add__(ArrayWrapper([3,4])) == 10  # sum operation
assert str(ArrayWrapper([23,98,42,70])) == "[23,98,42,70]"    # string operation
assert ArrayWrapper([]).__add__(ArrayWrapper([])) == 0         # sum of empty arrays

# Additional cases
assert ArrayWrapper([0,0,0]).__add__(ArrayWrapper([0,0])) == 0  # all zeros
assert str(ArrayWrapper([])) == "[]"                             # empty array string
assert ArrayWrapper([1000]*1000).__add__(ArrayWrapper([1000]*1000)) == 2000000  # max values
assert str(ArrayWrapper([1])) == "[1]"                            # single element
Test Why
sum of small arrays validates basic addition
string of multiple elements validates correct string format
sum of empty arrays checks edge case of empty input
sum of all zeros ensures zeros handled correctly
string of empty array ensures empty array formatting
sum of max size arrays stress test for large input
string of single element handles minimal non-empty array

Edge Cases

An important edge case is the empty array. Adding two empty arrays should return 0, and the string representation should be "[]". Our implementation naturally handles this because summing an empty list returns 0 in Python, and iterating over an empty slice in Go also results in 0.

Another edge case is arrays with a single element. This can reveal off-by-one errors in string conversion or addition. Our implementation works because both summing and joining a single element array is naturally handled by the built-in functions.

A third edge case is arrays at the maximum allowed length or element value. In Python, integers are unbounded, so the sum does not overflow, but in Go, large sums may need int type consideration. Our Go implementation assumes int can store the sum safely, which is correct for the given constraints (1000 elements * 1000 maximum value = 1,000,000). This edge case ensures the algorithm scales correctly.