Find All Numbers Disappeared In An Array Leetcode Problem 448 [Python]

If you're new to programming and looking to improve your coding skills, you're in the right place.

In this post, we'll walk through the Find All Numbers Disappeared In An Array problem, a LeetCode question.

We'll provide a beginner-friendly Python solution and delve into the reasoning behind our approach.

Let's get started!

Problem Overview

The problem statement is as follows: Given an array nums of n integers where nums[i] is in the range [1, n], return an array of all the integers in the range [1, n] that do not appear in nums.

Let's break it down with an example:

Example 1:

Input: nums = [4,3,2,7,8,2,3,1]
Output: [5,6]

In this example, the input array contains values from 1 to 8, but the numbers 5 and 6 are missing.

Constraints:

• n is the length of nums.
• 1 <= n <= 105
• 1 <= nums[i] <= n

To tackle this problem, we need to come up with an efficient algorithm that won't exceed O(1) memory usage.

Efficient Python Code Solution

def findDisappearedNumbers(nums):
    for n in nums:
        i = abs(n) - 1
        nums[i] = -1 * abs(nums[i])

    res = []
    for i, n in enumerate(nums):
        if n > 0:
            res.append(i + 1)
    return res

Now, let's break down the code step by step.

Phase 1: Mark the Existing Values

In the first phase, we iterate through the nums array and mark the values that exist by changing their signs.

This is done by mapping each value n to the corresponding index i, where i = abs(n) - 1.

We use the absolute value to handle negative numbers if they occur.

Then, we set nums[i] to be negative, or -abs(nums[i]).

This phase ensures that we identify which values exist in the input array efficiently without using extra memory.

Phase 2: Build the Result

In the second phase, we construct the result array that contains the missing values.

We iterate through the modified nums array using enumerate, which provides both the index and value.

If the value is positive, it means the corresponding index is missing in the original array, so we append i + 1 to the result array.

Time and Space Complexity

Now, let's analyze the time and space complexity of our solution.

• Time Complexity: O(n)
  • In the first phase, we iterate through the nums array once, marking the existing values.

This phase has a time complexity of O(n).
– In the second phase, we iterate through the modified nums array once to build the result.

This phase also has a time complexity of O(n).
– Overall, our solution has a time complexity of O(n).

• Space Complexity: O(1)
  • We use a constant amount of additional memory.

The result array is not considered extra memory in this context.

Reasoning Behind Our Approach

The key idea behind this solution is to use the input array itself to mark the existing values, without using extra memory.

We take advantage of the fact that every value in the input array is positive and maps to a unique index, which allows us to efficiently mark and identify the existing values.

By following this approach, we achieve an optimal solution in terms of memory usage and runtime.

This is a great example of how creative thinking can lead to efficient coding solutions.

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Conclusion

In this blog post, we've addressed the LeetCode problem Find All Numbers Disappeared In An Array (Problem 448) in a beginner-friendly manner.

We provided a Python solution that efficiently identifies the missing values without using extra memory.

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For the original problem and more details, check out the Find All Numbers Disappeared in an Array problem on LeetCode.

Happy coding!