Given string str of length N and an integer K, the task is to return the largest string in Dictionary Order by erasing K characters from that string.
A largest string Dictionary order is the last string when strings are arranged in alphabetical order.
Input: str = “ritz” K = 2
There are 6 possible ways of deleting two characters from s: “ri”, “rt”, “rz”, “it”, “iz”, “tz”.
Among these strings “tz” is the largest in dictionary order.
Thus “tz” is the desired output.
Input: str = “jackie” K = 2
The characters “a” and “c” are deleted to get the largest possible string.
Naive Approach: The idea is to find all the subsequence of the given string length N – K. Store those subsequences in a list. There will be nCm Such sequences. After the above steps print the largest string in alphabetical order stored in the list.
Time Complexity: O(2N-K)
Efficient Approach: The idea is to use a Deque. Below are the steps:
- Store all the characters of the string in the deque.
- Traverse the given string and for each character in the string keep popping the characters from deque if it is less than the last character stored in the deque. Perform this operation until K is non-zero.
- Now after the above operations, insert the current character in the deque.
- After the above operations, the string formed by the characters stored in the deque is the resultant string.
Below is the implementation of the above approach:
Time Complexity: O(N)
Auxiliary Space: O(N)