solves multiply strings

Author: anishLearnsToCode

Diff for: README.md

@@ -44,6 +44,7 @@
 |  38  | [Count and Say](https://leetcode.com/problems/count-and-say)                                                                                              |                                       [![Java](assets/java.png)](src/CountAndSay.java) [![Python](assets/python.png)](python/count_and_say.py)                                       |                                                                                                                                   |
 |  39  | [Combination Sum](https://leetcode.com/problems/combination-sum)                                                                                          |                                                                 [![Java](assets/java.png)](src/CombinationSum.java)                                                                  |                                                                                                                                   |
 |  40  | [Combination Sum II](https://leetcode.com/problems/combination-sum-ii)                                                                                    |                                                                [![Java](assets/java.png)](src/CombinationSumII.java)                                                                 |                                                                                                                                   |
+|  43  | [Multiply Strings](https://leetcode.com/problems/multiply-strings)                                                                                        |                                                                 [![Java](assets/java.png)](src/MultiplyStrings.java)                                                                 |                                                                                                                                   |
 |  53  | [Maximum SubArray](https://leetcode.com/problems/maximum-subarray)                                                                                        |                                 [![Java](assets/java.png)](src/MaximumSubArray.java) [![Python](assets/python.png)](python/maximum_sum_subarray.py)                                  |                                                                                                                                   |
 |  58  | [Length of Last Word](https://leetcode.com/problems/length-of-last-word)                                                                                  |                                 [![Java](assets/java.png)](src/LengthOfLastWord.java) [![Python](assets/python.png)](python/length_of_last_word.py)                                  |                                                                                                                                   |
 |  66  | [Plus One](https://leetcode.com/problems/plus-one)                                                                                                        |                                           [![Java](assets/java.png)](src/PlusOne.java) [![Python](assets/python.png)](python/plus_one.py)                                            |                                                                                                                                   |

Diff for: src/MultiplyStrings.java

@@ -0,0 +1,49 @@
+// https://leetcode.com/problems/multiply-strings
+// T: O(|num1| * |num2|)
+// S: O(|num1| + |num2|)
+
+public class MultiplyStrings {
+    private static final String ZERO = "0";
+    private static final String ONE = "1";
+
+    public String multiply(String num1, String num2) {
+        if (ZERO.equals(num1) || ZERO.equals(num2)) return ZERO;
+        if (ONE.equals(num1)) return num2;
+        if (ONE.equals(num2)) return num1;
+
+        StringBuilder result = new StringBuilder(ZERO);
+        for (int i = num2.length() - 1 ; i >= 0 ; i--) {
+            result = add(result, multiply(num1, num2.charAt(i) - '0', num2.length() - i - 1));
+        }
+        return result.toString();
+    }
+
+    private StringBuilder multiply(String number, int digit, int factor) {
+        final StringBuilder result = new StringBuilder();
+        int carry = 0;
+        for (int i = number.length() - 1 ; i >= 0 ; i--) {
+            int val = (number.charAt(i) - '0') * digit + carry;
+            result.append(val % 10);
+            carry = val / 10;
+        }
+        if (carry > 0) result.append(carry);
+        return result.reverse().append(ZERO.repeat(factor));
+    }
+
+    private StringBuilder add(StringBuilder a, StringBuilder b) {
+        final StringBuilder result = new StringBuilder();
+        int carry = 0;
+        for (int i = a.length() - 1, j = b.length() - 1 ; i >= 0 || j >= 0 ; i--, j--) {
+            int val = getDigit(a, i) + getDigit(b, j) + carry;
+            result.append(val % 10);
+            carry = val / 10;
+        }
+        if (carry > 0) result.append(carry);
+        return result.reverse();
+    }
+
+    private int getDigit(StringBuilder string, int index) {
+        if (index < 0) return 0;
+        return string.charAt(index) - '0';
+    }
+}