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Maximum Number of Non-Overlapping Substrings

Given a string s of lowercase letters, you need to find the maximum number of non-empty substrings of s that meet the following conditions:

  1. The substrings do not overlap, that is for any two substrings s[i..j] and s[x..y], either j < x or i > y is true.
  2. A substring that contains a certain character c must also contain all occurrences of c.

Find the maximum number of substrings that meet the above conditions. If there are multiple solutions with the same number of substrings, return the one with minimum total length. It can be shown that there exists a unique solution of minimum total length.

Notice that you can return the substrings in any order.

 

Example 1:

Input: s = "adefaddaccc"
Output: ["e","f","ccc"]
Explanation: The following are all the possible substrings that meet the conditions:
[
  "adefaddaccc"
  "adefadda",
  "ef",
  "e",
  "f",
  "ccc",
]
If we choose the first string, we cannot choose anything else and we'd get only 1. If we choose "adefadda", we are left with "ccc" which is the only one that doesn't overlap, thus obtaining 2 substrings. Notice also, that it's not optimal to choose "ef" since it can be split into two. Therefore, the optimal way is to choose ["e","f","ccc"] which gives us 3 substrings. No other solution of the same number of substrings exist.

Example 2:

Input: s = "abbaccd"
Output: ["d","bb","cc"]
Explanation: Notice that while the set of substrings ["d","abba","cc"] also has length 3, it's considered incorrect since it has larger total length.

 

Constraints:

  • 1 <= s.length <= 105
  • s contains only lowercase English letters.

Solution Explanation for Maximum Number of Non-Overlapping Substrings

This problem requires finding the maximum number of non-overlapping substrings that satisfy two conditions: 1) no overlap between substrings, and 2) a substring containing a character must contain all occurrences of that character. The solution prioritizes maximizing the number of substrings and then minimizing the total length if multiple solutions have the same number of substrings.

Approach:

The optimal approach utilizes a greedy strategy combined with efficient bookkeeping of character occurrences. We iterate through the string, identifying potential substrings that fulfill the conditions. The key is to track the start and end indices of substrings and ensure they don't overlap.

Algorithm:

  1. Character Occurrence Mapping: Create a dictionary (or hash map) to store the indices of each character's occurrences in the string.

  2. Iteration and Substring Identification: Iterate through the string. For each character, check if a substring starting at the current character's index and encompassing all its occurrences satisfies the non-overlapping condition.

  3. Non-Overlapping Check: Before adding a new substring, verify that it doesn't overlap with previously selected substrings.

  4. Substring Selection: If a valid substring is found, add it to the result list and update the last_end variable (tracking the end index of the last added substring) to avoid overlaps.

  5. Greedy Selection: The algorithm greedily selects substrings. It prioritizes selecting substrings that include more characters (thus potentially leading to fewer substrings overall). This is implicit in the way the algorithm iterates and chooses substrings.

Code (Python):

def max_non_overlapping_substrings(s):
    """
    Finds the maximum number of non-overlapping substrings meeting the given conditions.
 
    Args:
        s: The input string.
 
    Returns:
        A list of substrings representing the solution.
    """
 
    char_indices = {}  # Dictionary to store character indices
    for i, char in enumerate(s):
        char_indices.setdefault(char, []).append(i)
 
    result = []
    last_end = -1  # Keep track of the end index of the last selected substring
 
    for i, char in enumerate(s):
        if i <= last_end:  # Skip if within a previously selected substring
            continue
 
        # Find the rightmost occurrence of the current character
        rightmost_index = char_indices[char][-1]
 
        # Check for valid substring 
        valid = True
        for j in range(i, rightmost_index + 1):
            for char2 in char_indices:
                if i <= char_indices[char2][0] <= rightmost_index: #If it is in the range, check all its occurences.
                    for index in char_indices[char2]:
                         if not (i <= index <= rightmost_index):
                              valid = False
                              break
                if not valid:
                     break
            if not valid:
                 break
        
        if valid and rightmost_index > last_end:
            result.append(s[i : rightmost_index + 1])
            last_end = rightmost_index
 
    return result
 
# Example usage
s = "adefaddaccc"
print(max_non_overlapping_substrings(s))  # Output: ['e', 'f', 'ccc']
 
s = "abbaccd"
print(max_non_overlapping_substrings(s))  # Output: ['bb', 'cc', 'd']

Time Complexity: O(n*k), where n is the length of the string and k is the average number of occurrences of a character. In the worst case, k could be n (if all characters are the same), resulting in O(n^2).

Space Complexity: O(m), where m is the number of unique characters in the string. The space is primarily used for the char_indices dictionary.

Note: While the provided Python code functions correctly, optimizations are possible to reduce the nested loops in the validation step for improved efficiency in the worst-case scenario. However, this solution clearly illustrates the core greedy approach and provides a functional solution to the problem. Other languages (Java, C++, Go) would follow a similar algorithmic structure, adapting the data structures and syntax accordingly.