A common problem in programming arises when you need to find the largest possible number by removing k digits from a given number. Let us delve into Java’s method to find the largest number if to remove k digits from a source number.
1. Using the Arithmetic approach
Below is the code snippet to find the largest number possible after removing k digits from a given number using arithmetic operations.
package com.jcg.example;
public class LargestNumberAfterRemovingKDigits {
public static String removeKDigits(String num, int k) {
// Base case: If the number of digits to be removed is equal to the length of the number, return "0".
if (k == num.length()) {
return "0";
}
StringBuilder result = new StringBuilder();
int n = num.length();
int keep = n - k; // Number of digits to keep
// Greedy algorithm to find the largest number
for (char digit : num.toCharArray()) {
while (k > 0 && result.length() > 0 && result.charAt(result.length() - 1) < digit) {
result.deleteCharAt(result.length() - 1);
k--;
}
result.append(digit);
}
// Remove extra digits if k > 0
result.setLength(keep);
// Remove leading zeros
int startIndex = 0;
while (startIndex < result.length() - 1 && result.charAt(startIndex) == '0') {
startIndex++;
}
return result.substring(startIndex);
}
public static void main(String[] args) {
String num = "1432219";
int k = 3;
String largestNumber = removeKDigits(num, k);
System.out.println("Largest number after removing " + k + " digits: " + largestNumber);
}
}
1.1 Code Breakdown
- removeKDigits Method:
- The method takes a string representation of a number and an integer
krepresenting the number of digits to remove. - A base case checks if
kis equal to the length of the number, in which case “0” is returned. - We initialize a
StringBuilderto store the result and calculate the number of digits to keep. - A greedy algorithm iterates through each digit of the number, removing digits until we’ve removed
kdigits or the last digit is smaller than the current digit. - After the loop, any remaining digits to remove are removed from the end of the result.
- Leading zeros are removed from the result before returning it.
- The method takes a string representation of a number and an integer
- Main Method:
- The
mainmethod demonstrates the usage of theremoveKDigitsmethod with a sample input.
- The
1.2 Output
The output of the provided Java code will be:
Largest number after removing 3 digits: 4329
2. Using the Stack approach
Below is the code snippet to find the largest number possible after removing k digits from a given number using stack operations.
package com.jcg.example;
import java.util.Stack;
public class LargestNumberAfterRemovingKDigits {
public static String removeKDigits(String num, int k) {
Stack<Character> stack = new Stack<>();
// Remove digits from the number
for (char digit : num.toCharArray()) {
while (k > 0 && !stack.isEmpty() && stack.peek() < digit) {
stack.pop();
k--;
}
stack.push(digit);
}
// Remove remaining digits if k > 0
while (k > 0) {
stack.pop();
k--;
}
// Construct the result string
StringBuilder result = new StringBuilder();
while (!stack.isEmpty()) {
result.insert(0, stack.pop());
}
// Remove leading zeros
int startIndex = 0;
while (startIndex < result.length() - 1 && result.charAt(startIndex) == '0') {
startIndex++;
}
return result.substring(startIndex);
}
public static void main(String[] args) {
String num = "1432219";
int k = 3;
String largestNumber = removeKDigits(num, k);
System.out.println("Largest number after removing " + k + " digits: " + largestNumber);
}
}
2.1 Code Breakdown
- removeKDigits Method:
- The method takes a string representation of a number and an integer
krepresenting the number of digits to remove. - We initialize a stack to store digits.
- We iterate through each digit of the number and use a stack to remove digits while maintaining the highest possible number.
- The remaining digits are removed if
kis greater than 0. - The result string is constructed by popping digits from the stack.
- Leading zeros are removed from the result before returning it.
- The method takes a string representation of a number and an integer
- Main Method:
- The
mainmethod demonstrates the usage of theremoveKDigitsmethod with a sample input.
- The
2.2 Output
The output of the provided Java code will be:
Largest number after removing 3 digits: 4329
3. Conclusion
The code adeptly determines the largest number attainable by selectively removing k digits from a given number through arithmetic operations, showcasing its efficiency in optimizing the output. Additionally, it employs a stack data structure to achieve the same objective, highlighting its versatility in handling different approaches to the problem.
