415. Add Strings.md

415. Add Strings

---

leetcode-cn Daily Challenge on August 3rd, 2020.

---

Difficulty : Easy

Related Topics : String

---

Given two non-negative integers 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.

---

Solution

• mine
  • Java
    • Runtime: 2 ms, faster than 96.67%, Memory Usage: 39.6 MB, less than 33.03% of Java online submissions

      // O(N)time
      // O(N)space
      public String addStrings(String num1, String num2) {
          char[] arr1 = num1.toCharArray();
          char[] arr2 = num2.toCharArray();
          int i = arr1.length, j = arr2.length;
          char[] res = new char[Math.max(i, j) + 1];
          int t = 0;
          int k = 0;
          while (i > 0 || j > 0) {
              char c1 = i > 0 ? arr1[i - 1] : '0';
              char c2 = j > 0 ? arr2[j - 1] : '0';
              int sum = c1 - '0' + c2 - '0' + t;
              t = sum / 10;
              res[k++] = (char) (sum % 10 + '0');
              i--;
              j--;
          }
          if (t != 0) res[k++] = (char) (t + '0');
          StringBuilder sb = new StringBuilder();
          while (k > 0) {
              k--;
              sb.append(res[k]);
          }
          return sb.toString();
      }
      

---