Adding java solution for 43-Multiply-Strings

java/43-Multiply-Strings.java

@@ -0,0 +1,55 @@
+class Solution {
+    public String multiply(String num1, String num2) {
+
+        if ("0".equals(num1) || "0".equals(num2)) {
+            return "0";
+        }
+
+        int[] res = new int[num1.length() + num2.length()];
+
+        num1 = reverseString(num1);
+        num2 = reverseString(num2);
+
+        for (int i = 0; i < num1.length(); i++) {
+            for (int j = 0; j < num2.length(); j++) {
+                int digit = Integer.valueOf(String.valueOf(num1.charAt(i))) * Integer.valueOf(String.valueOf(num2.charAt(j)));
+                res[i + j] += digit;
+                res[i + j + 1] += res[i + j] / 10;
+                res[i + j] = res[i + j] % 10;
+            }
+        }
+
+        reverseArrayInPlace(res);
+
+        // Get the proper starting point to avoid leading zeros
+        int startIndex = 0;
+        while (startIndex < res.length) {
+            if (res[startIndex] != 0) {
+                break;
+            }
+            startIndex++;
+        }
+
+        StringBuilder buildResponse = new StringBuilder();
+        for (int i = startIndex; i < res.length; i++) {
+            buildResponse.append(res[i]);
+        }
+
+        return buildResponse.toString();
+    }
+
+    private String reverseString(String str) {
+        StringBuilder reversedStr = new StringBuilder(str);
+        reversedStr.reverse();
+        return reversedStr.toString();
+    }
+
+    private void reverseArrayInPlace(int[] arr) {
+        for (int i = 0; i < arr.length / 2; i++) {
+            int temp = arr[i];
+            arr[i] = arr[arr.length - i - 1];
+            arr[arr.length - i - 1] = temp;
+        }
+    }
+
+}