Create add-strings.py

Python/add-strings.py

@@ -0,0 +1,36 @@
+# Time:  O(n)
+# Space: O(1)
+
+# Given two non-negative numbers num1 and num2 represented as string,
+# return the sum of num1 and num2.
+#
+# Note:
+#
+# The length of both num1 and num2 is < 5100.
+# Both num1 and num2 contains only digits 0-9.
+# Both num1 and num2 does not contain any leading zero.
+# You must not use any built-in BigInteger library or
+# convert the inputs to integer directly.
+
+class Solution(object):
+    def addStrings(self, num1, num2):
+        """
+        :type num1: str
+        :type num2: str
+        :rtype: str
+        """
+        result = []
+        i, j, carry = len(num1) - 1, len(num2) - 1, 0
+
+        while i >= 0 or j >= 0 or carry:
+            if i >= 0:
+                carry += ord(num1[i]) - ord('0');
+                i -= 1
+            if j >= 0:
+                carry += ord(num2[j]) - ord('0');
+                j -= 1
+            result.append(str(carry % 10))
+            carry /= 10
+        result.reverse()
+
+        return "".join(result)