Lexicographically Minimum String After Removing Stars in Java
The problem involves determining the lexicographically minimum string after removing characters specified by a special symbol, often represented as a star ('*'). The task involves iterating through the string and applying a rule where each star symbol removes the preceding character. The primary objective is to process the string, maintain the order and integrity of the remaining characters, and construct the smallest possible string.
Example1:
Input: "erase*****"
Output: ""
Explanation: 'e', 'r', 'a', 's', 'e' are pushed onto the stack. Five stars remove the five characters one by one. The stack is empty after all operations.
Example2:
Input: "abc*de**f"
Output: "abf"
Explanation: 'a', 'b', 'c' are pushed onto the stack. The first star removes 'c'. 'd', 'e' are pushed onto the stack. Two stars remove 'e' and 'd'. 'f' is pushed onto the stack. The stack contains ['a',’b’, 'f'] at the end.
Example3:
Input: : "a*b*c*d*e*f"
Output: f
Explanation: Each '*' removes the preceding character from consideration. After processing the entire string, only 'f' remains in consideration since all other characters are removed due to preceding '*'. Therefore, the output is "f", which is the lexicographically minimum string after all specified removals.
Using Naive approach
Algorithm
Step 1: Initialize a StringBuilder with input string for modifications.
Step 2: Iterate through each character in the StringBuilder.
Step 3: Check if it's not the first character, remove the preceding character, adjust the index, remove the star at the beginning, and decrement the index.
Step 4: Convert the modified StringBuilder back to a string.
Step 5: Return the resultant string as the lexicographically minimum string after all specified removals.
Here is an implementation of Lexicographically Minimum String After Removing Stars using a naive approach:
FileName:LexicographicallyMinimumString.java
public class LexicographicallyMinimumString { // Function to compute the lexicographically minimum string after removing stars public static String minStringAfterRemovingStars(String s) { StringBuilder result = new StringBuilder(); for (int i = 0; i < s.length(); i++) { char currentChar = s.charAt(i); if (currentChar == '*') { if (result.length() > 0) { result.deleteCharAt(result.length() - 1); } } else { result.append(currentChar); } } return result.toString(); } public static void main(String[] args) { String s = "a*b*c*d*e*f"; System.out.println(minStringAfterRemovingStars(s)); } }
Output:
f
Complexity analysis: The naive approach implemented above has an overall time complexity of O(n2). Iterating through the string `result` character by character, each removal operation potentially causes a shift of characters, resulting in O(n2) time for each deletion. The overall space complexity is O(n) linear with respect to the input string length. Efficiency can degrade for larger inputs or multiple removal operations.
Using Optimized approach
Algorithm
Step 1: Create an empty stack for character storage.
Step 2: Iterate through characters for each character in the input string.
Step 3: If c is a star ('*'), pop the top character from the stack.
Step 4: If not a star, push c onto the stack.
Step 5: Create a StringBuilder for the final result string.
Step 6: Append each character to the StringBuilder.
Step 7: Convert the StringBuilder to a string and return it as the lexicographically minimum string.
Here is an implementation of Lexicographically Minimum String After Removing Stars using an optimized approach:
FileName:LexicographicallyMinimumString1.java
import java.util.Stack; public class LexicographicallyMinimumString1 { // Function to compute the lexicographically minimum string after removing stars public static String minStringAfterRemovingStars(String s) { Stackstack = new Stack<>(); // Traverse each character in the string for (char c : s.toCharArray()) { if (c == '*') { if (!stack.isEmpty()) { stack.pop(); } } else { stack.push(c); } } StringBuilder result = new StringBuilder(); // Build the result string from the characters remaining in the stack for (char c : stack) { result.append(c); } return result.toString(); } public static void main(String[] args) { String s = "a*b*c*d*e*f"; System.out.println(minStringAfterRemovingStars(s)); } }
Output:
f
Complexity analysis: The above approach has an overall time complexity of O(n) due to iterating through each character of the string `s` exactly once, making it linear with respect to the input string's size. The space complexity comes from the stack and the `StringBuilder` used to construct the result string, resulting in an overall space complexity of O(n) for processing moderate-sized strings within reasonable time and space limits.