Code to find largest number after removing K digits from a number (#16060)

Co-authored-by: Neetika Khandelwal <kwal.neetika2398@gmail.com>

algorithms-modules/algorithms-miscellaneous-7/src/main/java/com/baeldung/algorithms/largestNumberRemovingK/LargestNumberRemoveKDigits.java

@@ -0,0 +1,57 @@
+package com.baeldung.algorithms.largestNumberRemovingK;
+
+import java.util.*;
+
+public class LargestNumberRemoveKDigits {
+    public static int findLargestNumberUsingArithmetic(int num, int k) {
+        for (int j = 0; j < k; j++) {
+
+            int result = 0;
+            int i = 1;
+
+            while (num / i > 0) {
+                int temp = (num / (i * 10))
+                        * i
+                        + (num % i);
+                i *= 10;
+
+                result = Math.max(result, temp);
+            }
+            num = result;
+        }
+
+        return num;
+    }
+
+    public static int findLargestNumberUsingStack(int num, int k) {
+        String numStr = Integer.toString(num);
+        int length = numStr.length();
+
+        if (k == length) return 0;
+
+        Stack<Character> stack = new Stack<>();
+
+        for (int i = 0; i < length; i++) {
+            char digit = numStr.charAt(i);
+
+            while (k > 0 && !stack.isEmpty() && stack.peek() < digit) {
+                stack.pop();
+                k--;
+            }
+
+            stack.push(digit);
+        }
+
+        while (k > 0) {
+            stack.pop();
+            k--;
+        }
+
+        StringBuilder result = new StringBuilder();
+        while (!stack.isEmpty()) {
+            result.insert(0, stack.pop());
+        }
+
+        return Integer.parseInt(result.toString());
+    }
+}

algorithms-modules/algorithms-miscellaneous-7/src/test/java/com/baeldung/algorithms/largestNumberRemovingK/LargestNumberRemoveKDigitsUnitTest.java

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+package com.baeldung.algorithms.largestNumberRemovingK;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+public class LargestNumberRemoveKDigitsUnitTest {
+
+    @Test
+    public void givenNumber_UsingArithmeticRemoveKDigits_thenReturnLargestNumber(){
+        Assertions.assertEquals(LargestNumberRemoveKDigits.findLargestNumberUsingArithmetic(9461, 1), 961);
+        Assertions.assertEquals(LargestNumberRemoveKDigits.findLargestNumberUsingArithmetic(463, 2), 6);
+        Assertions.assertEquals(LargestNumberRemoveKDigits.findLargestNumberUsingArithmetic(98625410, 6), 98);
+        Assertions.assertEquals(LargestNumberRemoveKDigits.findLargestNumberUsingArithmetic(20, 2), 0);
+        Assertions.assertEquals(LargestNumberRemoveKDigits.findLargestNumberUsingArithmetic(98989, 4), 9);
+    }
+
+    @Test
+    public void givenNumber_UsingStackRemoveKDigits_thenReturnLargestNumber(){
+        Assertions.assertEquals(LargestNumberRemoveKDigits.findLargestNumberUsingStack(9461, 1), 961);
+        Assertions.assertEquals(LargestNumberRemoveKDigits.findLargestNumberUsingStack(463, 2), 6);
+        Assertions.assertEquals(LargestNumberRemoveKDigits.findLargestNumberUsingStack(98625410, 6), 98);
+        Assertions.assertEquals(LargestNumberRemoveKDigits.findLargestNumberUsingStack(20, 2), 0);
+        Assertions.assertEquals(LargestNumberRemoveKDigits.findLargestNumberUsingStack(98989, 4), 9);
+    }
+}