feat(leetcode): add No.415

Author: mickey0524

415.Add-Strings.java

@@ -0,0 +1,54 @@
+// https://leetcode.com/problems/add-strings/
+//
+// algorithms
+// Easy (44.7%)
+// Total Accepted:    114,595
+// Total Submissions: 256,348
+
+
+class Solution {
+    public String addStrings(String num1, String num2) {
+        int len1 = num1.length(), len2 = num2.length();
+        if (len1 == 0) {
+            return num2;
+        }
+        if (len2 == 0) {
+            return num1;
+        }
+
+        LinkedList<Integer> l = new LinkedList<>();
+
+        int idx1 = 0, idx2 = 0, carry = 0;
+        while (idx1 < len1 || idx2 < len2) {
+            int tmp = carry;
+            if (idx1 < len1) {
+                tmp += (num1.charAt(len1 - 1 - idx1) - '0');
+                idx1++;
+            }
+            if (idx2 < len2) {
+                tmp += (num2.charAt(len2 - 1 - idx2) - '0');
+                idx2++;
+            }
+
+            if (tmp >= 10) {
+                carry = 1;
+                tmp -= 10;
+            } else {
+                carry = 0;
+            }
+
+            l.addFirst(tmp);
+        }
+        
+        if (carry == 1) {
+            l.addFirst(1);
+        }
+
+        StringBuilder sb = new StringBuilder();
+        for (int n : l) {
+            sb.append(n);
+        }
+
+        return sb.toString();
+    }
+}