Create 415. Add Strings.java

Author: futurus

415. Add Strings.java

@@ -0,0 +1,18 @@
+public class Solution {
+    public String addStrings(String num1, String num2) {
+        int i = num1.length()-1, j = num2.length()-1, carry = 0, num = 0, digit1 = 0, digit2 = 0;
+        StringBuilder sb = new StringBuilder();
+        
+        while (i >= 0 || j >= 0 || carry > 0) {
+            digit1 = i < 0 ? 0 : (num1.charAt(i) - '0');
+            digit2 = j < 0 ? 0 : (num2.charAt(j) - '0');
+            num = digit1 + digit2 + carry;
+            carry = num / 10;
+            num %= 10;
+            sb.insert(0, num);
+            i--; j--;
+        }
+        
+        return sb.toString();
+    }
+}