Find All Numbers Disappeared in an Array - Solution & Explanation

This approach uses the properties of the array and its indices to track missing numbers. By iterating through the array and using the values to mark the corresponding indices, we can identify which indices were never marked and thus determine the missing numbers.

Steps:

1. Iterate over each element in the array nums.
2. For each element, compute its corresponding index by subtracting 1 (as the array is zero-based).
3. Use this index to mark the original array as visited by negating the value at that index.
4. After processing all numbers, iterate over the array once more. The indices with positive values indicate the missing numbers, which are the result.

This C solution implements index marking by negating the values at positions corresponding to the numbers seen in the array. It iterates over the array twice: first to mark visited numbers and second to collect those which are still positive, indicating those numbers are missing.

In this approach, we utilize a set to only store unique numbers from 1 to n that appear in the input array. By comparing this set to the complete range of numbers, we can directly determine those not present in the input.

Steps:

1. Initialize a set containing all the numbers in nums.
2. Create a result list which will store numbers not present in the set, iterating from 1 to n.
3. For each number in the iteration, check if it is missing in the set and append to the result list if so.

In this solution, a boolean array acts as a set storing flags for numbers detected in the input. Subsequent traversal is made to seek indices flagged false, representing the missing numbers.