Update Add Strings.md

Author: cheatsheet1999

JS-Algo/Add Strings.md

@@ -11,17 +11,16 @@
  * @return {string}
  */
 var addStrings = function(num1, num2) {
-    let carry = 0;
     let sum = '';
-     
-    for (let i = num1.length - 1, j = num2.length - 1; i >= 0 || j >= 0 || carry > 0; i--, j--) { 
-        const digit1 = i < 0 ? 0 : num1.charAt(i) - '0';
-        const digit2 = j < 0 ? 0 : num2.charAt(j) - '0';
-        const digitsSum = digit1 + digit2 + carry;
-        sum = `${digitsSum % 10}${sum}`;
-        carry = Math.floor(digitsSum / 10);
+    let carry = 0;
+    for(let i = num1.length - 1, j = num2.length - 1; i >= 0 || j >= 0 || carry > 0; i--, j--) {
+        let digit1 = i < 0 ? 0 : num1.charAt(i) - '0';
+        let digit2 = j < 0 ? 0 : num2.charAt(j) - '0';
+        let currSum = digit1 + digit2 + carry;
+        // currSum % 10 cannot be currSum - 10, because the first digit may not larger than 10, then cause negative number
+        sum = `${currSum % 10}${sum}`;
+        carry = Math.floor(currSum / 10);
     }
-    
-    return sum;
+    return sum
 };
 ```