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Clear filtersNormalise the string (lower case, remove non-alphanumeric characters), then compare characters from both ends moving inward with two pointers. This is O(n) time and O(1) extra space beyond the normalised copy.
Walk through the array once while storing each number鈥檚 index in a hash map; for every number, check whether target minus that number is already in the map. This runs in O(n) time and O(n) space.
Recursively walk the array: when an element is an array, flatten it and append the result; otherwise append the element. An iterative version uses an explicit stack to avoid recursion limits.