Solution to "Minimum Number of Operations to Move All Balls to Each Box"

 problem

src/main/java/com/github/dkoval/leetcode/challenge/MinimumNumberOfOperationsToMoveAllBallsToEachBox.java

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+package com.github.dkoval.leetcode.challenge;
+
+/**
+ * <a href="https://leetcode.com/problems/minimum-number-of-operations-to-move-all-balls-to-each-box/">Minimum Number of Operations to Move All Balls to Each Box</a>
+ * <p>
+ * You have n boxes. You are given a binary string boxes of length n, where boxes[i] is '0' if the ith box is empty,
+ * and '1' if it contains one ball.
+ * <p>
+ * In one operation, you can move one ball from a box to an adjacent box. Box i is adjacent to box j if abs(i - j) == 1.
+ * Note that after doing so, there may be more than one ball in some boxes.
+ * <p>
+ * Return an array answer of size n, where answer[i] is the minimum number of operations needed to move all the balls
+ * to the ith box.
+ * <p>
+ * Each answer[i] is calculated considering the initial state of the boxes.
+ * <p>
+ * Constraints:
+ * <ul>
+ *  <li>n == boxes.length</li>
+ *  <li>1 <= n <= 2000</li>
+ *  <li>boxes[i] is either '0' or '1'.</li>
+ * </ul>
+ */
+public interface MinimumNumberOfOperationsToMoveAllBallsToEachBox {
+
+    int[] minOperations(String boxes);
+
+    // O(N^2) time | O(1) space
+    class MinimumNumberOfOperationsToMoveAllBallsToEachBoxRev1 implements MinimumNumberOfOperationsToMoveAllBallsToEachBox {
+
+        @Override
+        public int[] minOperations(String boxes) {
+            final var n = boxes.length();
+
+            final var ans = new int[n];
+            for (var i = 0; i < n; i++) {
+                for (var j = 0; j < n; j++) {
+                    if (boxes.charAt(j) == '1') {
+                        ans[i] += Math.abs(i - j);
+                    }
+                }
+            }
+            return ans;
+        }
+    }
+}

src/test/kotlin/com/github/dkoval/leetcode/challenge/MinimumNumberOfOperationsToMoveAllBallsToEachBoxTest.kt

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+package com.github.dkoval.leetcode.challenge
+
+import com.github.dkoval.leetcode.challenge.MinimumNumberOfOperationsToMoveAllBallsToEachBox.MinimumNumberOfOperationsToMoveAllBallsToEachBoxRev1
+import org.junit.jupiter.api.Assertions.assertArrayEquals
+import org.junit.jupiter.api.Nested
+import org.junit.jupiter.api.extension.ExtensionContext
+import org.junit.jupiter.params.ParameterizedTest
+import org.junit.jupiter.params.provider.Arguments
+import org.junit.jupiter.params.provider.ArgumentsProvider
+import org.junit.jupiter.params.provider.ArgumentsSource
+import java.util.stream.Stream
+
+internal class MinimumNumberOfOperationsToMoveAllBallsToEachBoxTest {
+
+    class InputArgumentsProvider : ArgumentsProvider {
+
+        override fun provideArguments(context: ExtensionContext): Stream<out Arguments> = Stream.of(
+            Arguments.of(
+                "110",
+                intArrayOf(1, 1, 3)
+            ),
+            Arguments.of(
+                "001011",
+                intArrayOf(11, 8, 5, 4, 3, 4)
+            )
+        )
+    }
+
+    @Nested
+    inner class MinimumNumberOfOperationsToMoveAllBallsToEachBoxRev1Test {
+
+        @ParameterizedTest
+        @ArgumentsSource(InputArgumentsProvider::class)
+        fun `should return an array answer of size n, where i-th answer is the minimum number of operations needed to move all the balls to the ith box`(
+            boxes: String,
+            expected: IntArray
+        ) {
+            MinimumNumberOfOperationsToMoveAllBallsToEachBoxRev1().test(boxes, expected)
+        }
+    }
+}
+
+private fun MinimumNumberOfOperationsToMoveAllBallsToEachBox.test(boxes: String, expected: IntArray) {
+    val actual = minOperations(boxes)
+    assertArrayEquals(expected, actual)
+}