Add AddStrings task solution

Author: andrei-punko

src/main/java/by/andd3dfx/numeric/AddStrings.java

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+package by.andd3dfx.numeric;
+
+/**
+ * <pre>
+ * <a href="https://leetcode.com/problems/add-strings/">Task description</a>
+ *
+ * Given two non-negative integers, num1 and num2 represented as string, return the sum of num1 and num2 as a string.
+ *
+ * You must solve the problem without using any built-in library for handling large integers (such as BigInteger).
+ * You must also not convert the inputs to integers directly.
+ *
+ * Example 1:
+ * Input: num1 = "11", num2 = "123"
+ * Output: "134"
+ *
+ * Example 2:
+ * Input: num1 = "456", num2 = "77"
+ * Output: "533"
+ *
+ * Example 3:
+ * Input: num1 = "0", num2 = "0"
+ * Output: "0"
+ * </pre>
+ */
+public class AddStrings {
+
+    public static String addStrings(String num1, String num2) {
+        var chars1 = num1.toCharArray();
+        var i = chars1.length - 1;
+
+        var chars2 = num2.toCharArray();
+        var j = chars2.length - 1;
+
+        var sb = new StringBuilder();
+        var accumulator = 0;
+        while (i >= 0 && j >= 0) {
+            int n1 = chars1[i] - '0';
+            int n2 = chars2[j] - '0';
+
+            var res = n1 + n2 + accumulator;
+            accumulator = 0;
+            if (res <= 9) {
+                sb.append(res);
+            } else {
+                sb.append(res % 10);
+                res = res / 10;
+                accumulator = res;
+            }
+
+            i--;
+            j--;
+        }
+        while (i >= 0) {
+            int n1 = chars1[i] - '0' + accumulator;
+            accumulator = 0;
+            sb.append(n1);
+            i--;
+        }
+        while (j >= 0) {
+            int n2 = chars2[j] - '0' + accumulator;
+            accumulator = 0;
+            sb.append(n2);
+            j--;
+        }
+
+        return sb.reverse().toString();
+    }
+}

src/test/java/by/andd3dfx/numeric/AddStringsTest.java

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+package by.andd3dfx.numeric;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.Test;
+
+public class AddStringsTest {
+
+    @Test
+    public void addStrings() {
+        assertThat(AddStrings.addStrings("11", "123")).isEqualTo("134");
+        assertThat(AddStrings.addStrings("456", "77")).isEqualTo("533");
+        assertThat(AddStrings.addStrings("0", "0")).isEqualTo("0");
+        assertThat(AddStrings.addStrings("79", "179")).isEqualTo("258");
+    }
+}