solve problem Add Strings

Author: P-ppc

README.md

@@ -127,6 +127,7 @@ All solutions will be accepted!
 |783|[Minimum Distance Between Bst Nodes](https://leetcode-cn.com/problems/minimum-distance-between-bst-nodes/description/)|[java/py/js](./algorithms/MinimumDistanceBetweenBstNodes)|Easy|
 |747|[Largest Number At Least Twice Of Others](https://leetcode-cn.com/problems/largest-number-at-least-twice-of-others/description/)|[java/py/js](./algorithms/LargestNumberAtLeastTwiceOfOthers)|Easy|
 |455|[assign cookies](https://leetcode-cn.com/problems/assign-cookies/description/)|[java/py/js](./algorithms/AssignCookies)|Easy|
+|415|[Add Strings](https://leetcode-cn.com/problems/add-strings/description/)|[java/py/js](./algorithms/AddStrings)|Easy|
 
 # Database
 |#|Title|Solution|Difficulty|

algorithms/AddStrings/README.md

@@ -0,0 +1,21 @@
+class Solution {
+    public String addStrings(String num1, String num2) {
+        String res = "";
+        int carry = 0,
+            i = num1.length() - 1,
+            j = num2.length() - 1;
+        
+        while (i >= 0 || j >= 0) {
+            int a = i >= 0 ? num1.charAt(i) - '0' : 0,
+                b = j >= 0 ? num2.charAt(j) - '0' : 0,
+                sum = a + b + carry;
+            
+            res = String.valueOf(sum % 10) + res;
+            carry = sum / 10;
+            i--;
+            j--;
+        }
+        
+        return carry == 1 ? "1" + res : res;
+    }
+}

algorithms/AddStrings/Solution.java

@@ -0,0 +1,24 @@
+/**
+ * @param {string} num1
+ * @param {string} num2
+ * @return {string}
+ */
+var addStrings = function(num1, num2) {
+    let res = '',
+        carry = 0,
+        i = num1.length - 1,
+        j = num2.length - 1
+    
+    while (i >= 0 || j >= 0) {
+        let a = i >= 0 ? num1[i] - '0' : 0,
+            b = j >= 0 ? num2[j] - '0' : 0,
+            sum = a + b + carry
+        
+        res = String(sum % 10) + res
+        carry = parseInt(sum / 10)
+        i--
+        j--
+    }
+    
+    return carry ? '1' + res : res
+};

algorithms/AddStrings/solution.js

@@ -0,0 +1,24 @@
+    class Solution(object):
+    def addStrings(self, num1, num2):
+        """
+        :type num1: str
+        :type num2: str
+        :rtype: str
+        """
+        res = ''
+        carry = False
+        i = len(num1) - 1
+        j = len(num2) - 1
+        
+        while i >= 0 or j >= 0:
+            a = ord(num1[i]) - ord('0') if i >= 0 else 0
+            b = ord(num2[j]) - ord('0') if j >= 0 else 0
+            sm = a + b + 1 if carry else a + b
+            
+            res = str(sm % 10) + res
+            carry = sm >= 10
+            
+            i -= 1
+            j -= 1
+        
+        return '1' + res if carry else res