Time: 8 ms (65.24%), Space: 6.9 MB (41.09%) - LeetHub

0415-add-strings/0415-add-strings.cpp

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+class Solution {
+public:
+    string addStrings(string num1, string num2) {
+        string ans = "";
+        int i1 = num1.size() - 1, i2 = num2.size() - 1, carry = 0;
+        while (i1 >= 0 || i2 >= 0 || carry > 0) {
+            if (i1 >= 0) {
+                carry += num1[i1] - '0';
+                i1 -= 1;
+            }
+            if (i2 >= 0) {
+                carry += num2[i2] - '0';
+                i2 -= 1;
+            }
+            ans += char(carry % 10 + '0');
+            carry /= 10;
+        }
+        reverse(ans.begin(), ans.end());
+        return ans;
+    }
+};