An array is given of n length, and we need to calculate the next greater element for each element in given array. If next greater element is not available in given array then we need to fill ‘_’ at that index place.
Input : 6 3 9 8 10 2 1 15 7 Output : 7 6 10 9 15 3 2 _ 8 Here every element of array has next greater element but at index 7, 15 is the greatest element of given array and no other element is greater from 15 so at the index of 15 we fill with '_' . Input : 13 6 7 12 Output : _ 7 12 13 Here, at index 0, 13 is the greatest value in given array and no other array element is greater from 13 so at index 0 we fill '_'.
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A simple solution is to use two loops nested. The outer loop picks all elements one by one and inner loop finds the next greater element by linearly searching from beginning to end.
7 6 10 9 15 3 2 _ 8
Time Complexity : O(n*n)
Auxiliary Space: O(1)
An efficient solution is to one by one insert elements in a set (A self balancing binary search tree). After inserting into set, we search elements. After we find iterator of the searched element, we move iterator to next (note that set stores elements in sorted order) to find element which is just greater.
7 6 10 9 15 3 2 _ 8
Time Complexity : O(n Log n). Note that self balancing search tree (implemented by set in C++) insert operations take O(Log n) time to insert and find.
Auxiliary Space: O(n)
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